mrnaterpg thanks!

First observation, 10.3.38-MariaDB is now over 5 years old and no longer supported. Although I don't believe this to be the cause of your issues, I do strongly suggest to upgrade to 10.11 (LTS)

A subdomain installation shouldn't have an impact, but it does cause an issue from time to time when extensions don't generate their base URL's correctly for example.

What I would suggest as a next step is to enable debug, and try to enable the extension again. This should give us output either in the console, Flarum UI, or in the logs (/storage/logs). If you can share that, I'll have a much better idea what is going on..

    Okay...I enabled debug and tried to enable fof upload. Here is the debug log..

    [2023-12-18 16:04:16] flarum.ERROR: Error: Class "FoF\Upload\Extend\SvgSanitizer" not found in /var/www/html/dcs_flarum/vendor/fof/upload/extend.php:112
    Stack trace:
    #0 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/Extension.php(373): require()
    #1 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/Extension.php(388): Flarum\Extension\Extension->getExtenders()
    #2 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/Extension.php(303): Flarum\Extension\Extension->getLifecycleExtenders()
    #3 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/ExtensionManager.php(216): Flarum\Extension\Extension->enable()
    #4 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/Command/ToggleExtensionHandler.php(36): Flarum\Extension\ExtensionManager->enable()
    #5 /var/www/html/dcs_flarum/vendor/illuminate/bus/Dispatcher.php(122): Flarum\Extension\Command\ToggleExtensionHandler->handle()
    #6 /var/www/html/dcs_flarum/vendor/illuminate/pipeline/Pipeline.php(128): Illuminate\Bus\Dispatcher->Illuminate\Bus\{closure}()
    #7 /var/www/html/dcs_flarum/vendor/illuminate/pipeline/Pipeline.php(103): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
    #8 /var/www/html/dcs_flarum/vendor/illuminate/bus/Dispatcher.php(132): Illuminate\Pipeline\Pipeline->then()
    #9 /var/www/html/dcs_flarum/vendor/illuminate/bus/Dispatcher.php(78): Illuminate\Bus\Dispatcher->dispatchNow()
    #10 /var/www/html/dcs_flarum/vendor/flarum/core/src/Api/Controller/UpdateExtensionController.php(43): Illuminate\Bus\Dispatcher->dispatch()
    #11 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/RouteHandlerFactory.php(41): Flarum\Api\Controller\UpdateExtensionController->handle()
    #12 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/ExecuteRoute.php(27): Flarum\Http\RouteHandlerFactory->Flarum\Http\{closure}()
    #13 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\ExecuteRoute->process()
    #14 /var/www/html/dcs_flarum/vendor/flarum/core/src/Api/Middleware/ThrottleApi.php(33): Laminas\Stratigility\Next->handle()
    #15 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Api\Middleware\ThrottleApi->process()
    #16 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(44): Laminas\Stratigility\Next->handle()
    #17 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\CheckCsrfToken->process()
    #18 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/ResolveRoute.php(69): Laminas\Stratigility\Next->handle()
    #19 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\ResolveRoute->process()
    #20 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/SetLocale.php(51): Laminas\Stratigility\Next->handle()
    #21 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\SetLocale->process()
    #22 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/AuthenticateWithHeader.php(58): Laminas\Stratigility\Next->handle()
    #23 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\AuthenticateWithHeader->process()
    #24 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(31): Laminas\Stratigility\Next->handle()
    #25 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\AuthenticateWithSession->process()
    #26 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(52): Laminas\Stratigility\Next->handle()
    #27 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\RememberFromCookie->process()
    #28 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Laminas\Stratigility\Next->handle()
    #29 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\StartSession->process()
    #30 /var/www/html/dcs_flarum/vendor/flarum/core/src/Api/Middleware/FakeHttpMethods.php(29): Laminas\Stratigility\Next->handle()
    #31 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Api\Middleware\FakeHttpMethods->process()
    #32 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Laminas\Stratigility\Next->handle()
    #33 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\ParseJsonBody->process()
    #34 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Laminas\Stratigility\Next->handle()
    #35 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\HandleErrors->process()
    #36 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/InjectActorReference.php(25): Laminas\Stratigility\Next->handle()
    #37 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\InjectActorReference->process()
    #38 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(75): Laminas\Stratigility\Next->handle()
    #39 /var/www/html/dcs_flarum/vendor/middlewares/request-handler/src/RequestHandler.php(84): Laminas\Stratigility\MiddlewarePipe->process()
    #40 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Middlewares\RequestHandler->process()
    #41 /var/www/html/dcs_flarum/vendor/middlewares/base-path-router/src/BasePathRouter.php(101): Laminas\Stratigility\Next->handle()
    #42 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Middlewares\BasePathRouter->process()
    #43 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Middleware/OriginalMessages.php(36): Laminas\Stratigility\Next->handle()
    #44 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Laminas\Stratigility\Middleware\OriginalMessages->process()
    #45 /var/www/html/dcs_flarum/vendor/middlewares/base-path/src/BasePath.php(73): Laminas\Stratigility\Next->handle()
    #46 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Middlewares\BasePath->process()
    #47 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Middleware/ProcessIp.php(24): Laminas\Stratigility\Next->handle()
    #48 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/Next.php(49): Flarum\Http\Middleware\ProcessIp->process()
    #49 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(75): Laminas\Stratigility\Next->handle()
    #50 /var/www/html/dcs_flarum/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(64): Laminas\Stratigility\MiddlewarePipe->process()
    #51 /var/www/html/dcs_flarum/vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(73): Laminas\Stratigility\MiddlewarePipe->handle()
    #52 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Server.php(45): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
    #53 /var/www/html/dcs_flarum/public/index.php(26): Flarum\Http\Server->listen()
    #54 {main}
    [2023-12-18 16:04:25] flarum.ERROR: Error: Class "FoF\Upload\Extend\SvgSanitizer" not found in /var/www/html/dcs_flarum/vendor/fof/upload/extend.php:112
    Stack trace:
    #0 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/Extension.php(373): require()
    #1 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/Extension.php(139): Flarum\Extension\Extension->getExtenders()
    #2 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/ExtensionManager.php(390): Flarum\Extension\Extension->extend()
    #3 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/ExtensionServiceProvider.php(31): Flarum\Extension\ExtensionManager->extend()
    #4 [internal function]: Flarum\Extension\ExtensionServiceProvider->Flarum\Extension\{closure}()
    #5 /var/www/html/dcs_flarum/vendor/flarum/core/src/Foundation/Application.php(329): call_user_func()
    #6 /var/www/html/dcs_flarum/vendor/flarum/core/src/Foundation/Application.php(270): Flarum\Foundation\Application->fireAppCallbacks()
    #7 /var/www/html/dcs_flarum/vendor/flarum/core/src/Foundation/InstalledSite.php(151): Flarum\Foundation\Application->boot()
    #8 /var/www/html/dcs_flarum/vendor/flarum/core/src/Foundation/InstalledSite.php(82): Flarum\Foundation\InstalledSite->bootLaravel()
    #9 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Server.php(59): Flarum\Foundation\InstalledSite->bootApp()
    #10 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Server.php(36): Flarum\Http\Server->safelyBootAndGetHandler()
    #11 /var/www/html/dcs_flarum/public/index.php(26): Flarum\Http\Server->listen()
    #12 {main}

      mrnaterpg I've seen someone else also report this recently. Having just peeked at the code, it appears to be all good there. Let me investigate a little closer...

        mrnaterpg Could you do me a favour please and try this? :

        (Please do these commands in order)

        composer remove fof/upload
php flarum migrate
php flarum cache:clear
composer require fof/upload
php flarum migrate
php flarum cache:clear
php flarum extension:enable fof-upload

            katos Here is the result when reloading the page after doing each command in that order:
            'Flarum encountered a boot error (Error)
            Class "FoF\Upload\Extend\SvgSanitizer" not found
            thrown in /var/www/html/dcs_flarum/vendor/fof/upload/extend.php on line 112
            Error: Class "FoF\Upload\Extend\SvgSanitizer" not found in /var/www/html/dcs_flarum/vendor/fof/upload/extend.php:112
            Stack trace:
            #0 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/Extension.php(373): require()
            #1 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/Extension.php(139): Flarum\Extension\Extension->getExtenders()
            #2 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/ExtensionManager.php(390): Flarum\Extension\Extension->extend()
            #3 /var/www/html/dcs_flarum/vendor/flarum/core/src/Extension/ExtensionServiceProvider.php(31): Flarum\Extension\ExtensionManager->extend()
            #4 [internal function]: Flarum\Extension\ExtensionServiceProvider->Flarum\Extension{closure}()
            #5 /var/www/html/dcs_flarum/vendor/flarum/core/src/Foundation/Application.php(329): call_user_func()
            #6 /var/www/html/dcs_flarum/vendor/flarum/core/src/Foundation/Application.php(270): Flarum\Foundation\Application->fireAppCallbacks()
            #7 /var/www/html/dcs_flarum/vendor/flarum/core/src/Foundation/InstalledSite.php(151): Flarum\Foundation\Application->boot()
            #8 /var/www/html/dcs_flarum/vendor/flarum/core/src/Foundation/InstalledSite.php(82): Flarum\Foundation\InstalledSite->bootLaravel()
            #9 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Server.php(59): Flarum\Foundation\InstalledSite->bootApp()
            #10 /var/www/html/dcs_flarum/vendor/flarum/core/src/Http/Server.php(36): Flarum\Http\Server->safelyBootAndGetHandler()
            #11 /var/www/html/dcs_flarum/public/index.php(26): Flarum\Http\Server->listen()
            #12 {main}'

                mrnaterpg hmm, ok - please disable it again for now and I will do some digging into this for you.

                  mrnaterpg Can you try this for me, sorry?

                  composer clear-cache
composer dump-autoload

                  then try and run:

                  composer require enshrined/svg-sanitize
php flarum cache:clear
php flarum migrate

                  and see if fof/upload then works for you please?

                    It must've been related to the MariaDB version. I updated my server so I could get mariadb updated an lo and behold the plugin seems to be working