Thanks for your reply, I'm sorry if I keep stumbling across troubles here.
I reinstalled flarum and edited composer.json so that all extensions are pointing to the beta.8 (they are all first party, it's a fresh install). When I proceed with installing (that is, adding db and admin data) I still have the same problem. This is my flarum-2020-05-20.log
[2020-05-20 08:35:15] production.ERROR: Less_Exception_Parser: File `fa-brands.less` not found. in variables.less in /var/www/html/vendor/oyejorge/less.php/lib/Less/Parser.php:2793
Stack trace:
#0 /var/www/html/vendor/oyejorge/less.php/lib/Less/Parser.php(471): Less_Parser->Error('File `fa-brands...')
#1 /var/www/html/vendor/oyejorge/less.php/lib/Less/Tree/Import.php(275): Less_Parser->parseFile('fa-brands.less', 'fa-brands.less', true)
#2 /var/www/html/vendor/oyejorge/less.php/lib/Less/Tree/Import.php(198): Less_Tree_Import->ParseImport('fa-brands.less', 'fa-brands.less', Object(Less_Environment))
#3 /var/www/html/vendor/oyejorge/less.php/lib/Less/Tree/Ruleset.php(248): Less_Tree_Import->compile(Object(Less_Environment))
#4 /var/www/html/vendor/oyejorge/less.php/lib/Less/Tree/Import.php(279): Less_Tree_Ruleset->evalImports(Object(Less_Environment))
#5 /var/www/html/vendor/oyejorge/less.php/lib/Less/Tree/Import.php(198): Less_Tree_Import->ParseImport('/var/www/html/v...', 'common', Object(Less_Environment))
#6 /var/www/html/vendor/oyejorge/less.php/lib/Less/Tree/Ruleset.php(248): Less_Tree_Import->compile(Object(Less_Environment))
#7 /var/www/html/vendor/oyejorge/less.php/lib/Less/Tree/Ruleset.php(235): Less_Tree_Ruleset->evalImports(Object(Less_Environment))
#8 /var/www/html/vendor/oyejorge/less.php/lib/Less/Tree/Ruleset.php(70): Less_Tree_Ruleset->PrepareRuleset(Object(Less_Environment))
#9 /var/www/html/vendor/oyejorge/less.php/lib/Less/Parser.php(199): Less_Tree_Ruleset->compile(Object(Less_Environment))
#10 /var/www/html/vendor/flarum/core/src/Frontend/Compiler/LessCompiler.php(86): Less_Parser->getCss()
#11 /var/www/html/vendor/flarum/core/src/Frontend/Compiler/RevisionCompiler.php(151): Flarum\Frontend\Compiler\LessCompiler->compile(Array)
#12 /var/www/html/vendor/flarum/core/src/Frontend/Compiler/RevisionCompiler.php(81): Flarum\Frontend\Compiler\RevisionCompiler->save('forum-00c2b691....', Array)
#13 /var/www/html/vendor/flarum/core/src/Frontend/Compiler/RevisionCompiler.php(126): Flarum\Frontend\Compiler\RevisionCompiler->commit()
#14 /var/www/html/vendor/flarum/core/src/Frontend/Content/Assets.php(71): Flarum\Frontend\Compiler\RevisionCompiler->getUrl()
#15 [internal function]: Flarum\Frontend\Content\Assets->Flarum\Frontend\Content\{closure}(Object(Flarum\Frontend\Compiler\LessCompiler))
#16 /var/www/html/vendor/flarum/core/src/Frontend/Content/Assets.php(70): array_map(Object(Closure), Array)
#17 /var/www/html/vendor/flarum/core/src/Frontend/Content/Assets.php(54): Flarum\Frontend\Content\Assets->getUrls(Array)
#18 /var/www/html/vendor/flarum/core/src/Frontend/Frontend.php(65): Flarum\Frontend\Content\Assets->__invoke(Object(Flarum\Frontend\Document), Object(Laminas\Diactoros\ServerRequest))
#19 /var/www/html/vendor/flarum/core/src/Frontend/Frontend.php(57): Flarum\Frontend\Frontend->populate(Object(Flarum\Frontend\Document), Object(Laminas\Diactoros\ServerRequest))
#20 /var/www/html/vendor/flarum/core/src/Frontend/Controller.php(34): Flarum\Frontend\Frontend->document(Object(Laminas\Diactoros\ServerRequest))
#21 /var/www/html/vendor/flarum/core/src/Http/RouteHandlerFactory.php(40): Flarum\Frontend\Controller->handle(Object(Laminas\Diactoros\ServerRequest))
#22 /var/www/html/vendor/flarum/core/src/Http/Middleware/DispatchRoute.php(67): Flarum\Http\RouteHandlerFactory->Flarum\Http\{closure}(Object(Laminas\Diactoros\ServerRequest), Array)
#23 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\DispatchRoute->process(Object(Laminas\Diactoros\ServerRequest), Object(Closure))
#24 /var/www/html/vendor/flarum/core/src/Http/Middleware/ShareErrorsFromSession.php(58): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#25 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ShareErrorsFromSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#26 /var/www/html/vendor/flarum/core/src/Http/Middleware/SetLocale.php(51): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#27 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\SetLocale->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#28 /var/www/html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(34): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#29 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\AuthenticateWithSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#30 /var/www/html/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(52): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#31 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\RememberFromCookie->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#32 /var/www/html/vendor/flarum/core/src/Http/Middleware/StartSession.php(62): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#33 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\StartSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#34 /var/www/html/vendor/flarum/core/src/Http/Middleware/CollectGarbage.php(48): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#35 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\CollectGarbage->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#36 /var/www/html/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(29): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#37 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ParseJsonBody->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#38 /var/www/html/vendor/flarum/core/src/Http/Middleware/HandleErrorsWithView.php(67): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#39 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\HandleErrorsWithView->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#40 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#41 /var/www/html/vendor/middlewares/request-handler/src/RequestHandler.php(84): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#42 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\RequestHandler->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#43 /var/www/html/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#44 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePathRouter->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#45 /var/www/html/vendor/middlewares/base-path/src/BasePath.php(53): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#46 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePath->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#47 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#48 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(73): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\EmptyPipelineHandler))
#49 /var/www/html/vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(96): Laminas\Stratigility\MiddlewarePipe->handle(Object(Laminas\Diactoros\ServerRequest))
#50 /var/www/html/vendor/flarum/core/src/Http/Server.php(46): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
#51 /var/www/html/public/index.php(22): Flarum\Http\Server->listen()
#52 {main}
[2020-05-20 08:35:21] production.ERROR: Flarum\Http\Exception\MethodNotAllowedException: POST in /var/www/html/vendor/flarum/core/src/Http/Middleware/DispatchRoute.php:62
Stack trace:
#0 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\DispatchRoute->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#1 /var/www/html/vendor/flarum/core/src/Http/Middleware/ShareErrorsFromSession.php(58): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#2 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ShareErrorsFromSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#3 /var/www/html/vendor/flarum/core/src/Http/Middleware/SetLocale.php(51): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#4 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\SetLocale->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#5 /var/www/html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(34): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#6 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\AuthenticateWithSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#7 /var/www/html/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(52): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#8 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\RememberFromCookie->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#9 /var/www/html/vendor/flarum/core/src/Http/Middleware/StartSession.php(62): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#10 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\StartSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#11 /var/www/html/vendor/flarum/core/src/Http/Middleware/CollectGarbage.php(48): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#12 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\CollectGarbage->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#13 /var/www/html/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(29): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#14 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ParseJsonBody->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#15 /var/www/html/vendor/flarum/core/src/Http/Middleware/HandleErrorsWithView.php(67): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#16 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\HandleErrorsWithView->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#17 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#18 /var/www/html/vendor/middlewares/request-handler/src/RequestHandler.php(84): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#19 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\RequestHandler->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#20 /var/www/html/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#21 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePathRouter->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#22 /var/www/html/vendor/middlewares/base-path/src/BasePath.php(53): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#23 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePath->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#24 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#25 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(73): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\EmptyPipelineHandler))
#26 /var/www/html/vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(96): Laminas\Stratigility\MiddlewarePipe->handle(Object(Laminas\Diactoros\ServerRequest))
#27 /var/www/html/vendor/flarum/core/src/Http/Server.php(46): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
#28 /var/www/html/public/index.php(22): Flarum\Http\Server->listen()
#29 {main}
and flarum-installer.log
[2020-05-20 08:34:50] Flarum Installer.ERROR: ArgumentCountError: Too few arguments to function Illuminate\Filesystem\Filesystem::{closure}(), 1 passed and exactly 2 expected in /var/www/html/vendor/flarum/tags/migrations/2015_02_24_000000_create_tags_table.php:17
Stack trace:
#0 [internal function]: Illuminate\Filesystem\Filesystem->{closure}(Object(Illuminate\Database\Schema\MySqlBuilder))
#1 /var/www/html/vendor/flarum/core/src/Database/Migrator.php(197): call_user_func(Object(Closure), Object(Illuminate\Database\Schema\MySqlBuilder))
#2 /var/www/html/vendor/flarum/core/src/Database/Migrator.php(130): Flarum\Database\Migrator->runClosureMigration(Array)
#3 /var/www/html/vendor/flarum/core/src/Database/Migrator.php(113): Flarum\Database\Migrator->runUp('/var/www/html/p...', '2015_02_24_0000...', Object(Flarum\Extension\Extension))
#4 /var/www/html/vendor/flarum/core/src/Database/Migrator.php(87): Flarum\Database\Migrator->runMigrationList('/var/www/html/p...', Array, Object(Flarum\Extension\Extension))
#5 /var/www/html/vendor/flarum/core/src/Extension/ExtensionManager.php(238): Flarum\Database\Migrator->run('/var/www/html/p...', Object(Flarum\Extension\Extension))
#6 /var/www/html/vendor/flarum/core/src/Extension/ExtensionManager.php(127): Flarum\Extension\ExtensionManager->migrate(Object(Flarum\Extension\Extension))
#7 /var/www/html/vendor/flarum/core/src/Install/Console/InstallCommand.php(332): Flarum\Extension\ExtensionManager->enable('flarum-tags')
#8 /var/www/html/vendor/flarum/core/src/Install/Console/InstallCommand.php(204): Flarum\Install\Console\InstallCommand->enableBundledExtensions()
#9 /var/www/html/vendor/flarum/core/src/Install/Console/InstallCommand.php(113): Flarum\Install\Console\InstallCommand->install()
#10 /var/www/html/vendor/flarum/core/src/Console/AbstractCommand.php(39): Flarum\Install\Console\InstallCommand->fire()
#11 /var/www/html/vendor/symfony/console/Command/Command.php(255): Flarum\Console\AbstractCommand->execute(Object(Symfony\Component\Console\Input\StringInput), Object(Symfony\Component\Console\Output\StreamOutput))
#12 /var/www/html/vendor/flarum/core/src/Install/Controller/InstallController.php(94): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\StringInput), Object(Symfony\Component\Console\Output\StreamOutput))
#13 /var/www/html/vendor/flarum/core/src/Http/RouteHandlerFactory.php(40): Flarum\Install\Controller\InstallController->handle(Object(Laminas\Diactoros\ServerRequest))
#14 /var/www/html/vendor/flarum/core/src/Http/Middleware/DispatchRoute.php(67): Flarum\Http\RouteHandlerFactory->Flarum\Http\{closure}(Object(Laminas\Diactoros\ServerRequest), Array)
#15 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\DispatchRoute->process(Object(Laminas\Diactoros\ServerRequest), Object(Closure))
#16 /var/www/html/vendor/flarum/core/src/Http/Middleware/StartSession.php(62): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#17 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\StartSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#18 /var/www/html/vendor/flarum/core/src/Http/Middleware/HandleErrorsWithWhoops.php(43): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#19 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\HandleErrorsWithWhoops->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#20 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#21 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(73): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\EmptyPipelineHandler))
#22 /var/www/html/vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(96): Laminas\Stratigility\MiddlewarePipe->handle(Object(Laminas\Diactoros\ServerRequest))
#23 /var/www/html/vendor/flarum/core/src/Http/Server.php(46): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
#24 /var/www/html/public/index.php(22): Flarum\Http\Server->listen()
#25 {main}
I might as well add that I use mariadb Ver 15.1 Distrib 10.3.22-MariaDB
and PHP 7.3.14-1
on Debian 10, I think I meet all requirements so I can't figure out what I'm doing wrong...