Seems like a typo in code:
[2020-12-21 12:12:26] flarum.ERROR: ParseError: syntax error, unexpected ')' in /app/vendor/fof/polls/src/Api/Controllers/DeletePollController.php:54
Stack trace:
#0 /app/vendor/composer/ClassLoader.php(322): Composer\Autoload\includeFile('/app/vendor/com...')
#1 [internal function]: Composer\Autoload\ClassLoader->loadClass('FoF\\Polls\\Api\\C...')
#2 [internal function]: spl_autoload_call('FoF\\Polls\\Api\\C...')
#3 /app/vendor/illuminate/container/Container.php(803): ReflectionClass->__construct('FoF\\Polls\\Api\\C...')
#4 /app/vendor/illuminate/container/Container.php(681): Illuminate\Container\Container->build('FoF\\Polls\\Api\\C...')
#5 /app/vendor/illuminate/container/Container.php(629): Illuminate\Container\Container->resolve('FoF\\Polls\\Api\\C...', Array)
#6 /app/vendor/flarum/core/src/Http/RouteHandlerFactory.php(74): Illuminate\Container\Container->make('FoF\\Polls\\Api\\C...')
#7 /app/vendor/flarum/core/src/Http/RouteHandlerFactory.php(34): Flarum\Http\RouteHandlerFactory->resolveController('FoF\\Polls\\Api\\C...')
#8 /app/vendor/flarum/core/src/Http/Middleware/ExecuteRoute.php(27): Flarum\Http\RouteHandlerFactory->Flarum\Http\{closure}(Object(Laminas\Diactoros\ServerRequest), Array)
#9 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ExecuteRoute->process(Object(Laminas\Diactoros\ServerRequest), Object(Closure))
#10 /app/vendor/flarum/core/src/Api/Middleware/ThrottleApi.php(33): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#11 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Api\Middleware\ThrottleApi->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#12 /app/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(48): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#13 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\CheckCsrfToken->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#14 /app/vendor/flarum/core/src/Http/Middleware/ResolveRoute.php(67): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#15 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ResolveRoute->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#16 /app/vendor/flarum/core/src/Http/Middleware/SetLocale.php(50): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#17 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\SetLocale->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#18 /app/vendor/flarum/core/src/Http/Middleware/AuthenticateWithHeader.php(55): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#19 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\AuthenticateWithHeader->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#20 /app/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(32): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#21 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\AuthenticateWithSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#22 /app/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(51): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#23 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\RememberFromCookie->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#24 /app/vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#25 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\StartSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#26 /app/vendor/flarum/core/src/Api/Middleware/FakeHttpMethods.php(29): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#27 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Api\Middleware\FakeHttpMethods->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#28 /app/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#29 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ParseJsonBody->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#30 /app/vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#31 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\HandleErrors->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#32 /app/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#33 /app/vendor/middlewares/request-handler/src/RequestHandler.php(84): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#34 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\RequestHandler->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#35 /app/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#36 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePathRouter->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#37 /app/vendor/laminas/laminas-stratigility/src/Middleware/OriginalMessages.php(42): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#38 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Laminas\Stratigility\Middleware\OriginalMessages->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#39 /app/vendor/middlewares/base-path/src/BasePath.php(53): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#40 /app/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePath->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#41 /app/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#42 /app/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(73): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\EmptyPipelineHandler))
#43 /app/vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(96): Laminas\Stratigility\MiddlewarePipe->handle(Object(Laminas\Diactoros\ServerRequest))
#44 /app/vendor/flarum/core/src/Http/Server.php(42): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
#45 /app/public/index.php(22): Flarum\Http\Server->listen()
#46 {main}
beta.15, version 0.3.0, happens when trying to remove a poll.