Sorry for bumping in a few months old post, but I seem to be having the same problem.
PHP Fatal error: Uncaught ReflectionException: Class Flarum\Forum\UrlGenerator does not exist in /forum/vendor/illuminate/container/Container.php:838
Stack trace:
#0 /forum/vendor/illuminate/container/Container.php(838): ReflectionParameter->getClass()
#1 /forum/vendor/illuminate/container/Container.php(807): Illuminate\Container\Container->resolveDependencies(Array)
#2 /forum/vendor/illuminate/container/Container.php(658): Illuminate\Container\Container->build('Matpompili\\Favi...')
#3 /forum/vendor/illuminate/container/Container.php(609): Illuminate\Container\Container->resolve('Matpompili\\Favi...', Array)
#4 /forum/vendor/flarum/core/src/Foundation/Application.php(521): Illuminate\Container\Container->make('Matpompili\\Favi...', Array)
#5 /forum/vendor/illuminate/events/Dispatcher.php(154): Flarum\Foundation\Application->make('Matpompili\\Favi...')
#6 /forum/vendor/illuminate/events/Dispatcher.php(140): Illuminate\Event in /forum/vendor/illuminate/container/Container.php on line 838
This is the plugin I installed: https://flagrow.io/extensions/7h3ev1l/flarum-ext-theevil-silencebgsoftware-fork