SOLVED! I read at “ERR_BUILD_M3_CACHE: Failed to build cache for Mautic 3” that somebody had upgrade-problems and I found similar errors in my log today. Than the user @imrodmartin revealed that’s because of an outdated plugin. I deleted than the Plugin “MauticExtendeeToolsBundle” (which generated the errors) and now deleting the cache-folder is not a problem anymore.
For future searchers: The error was:
[27-Apr-2021 14:54:22 UTC] ReflectionException: Class "Mautic\CoreBundle\EventListener\CommonSubscriber" not found while loading "MauticPlugin\MauticExtendeeToolsBundle\EventListener\ButtonSubscriber". - in file /mywebsite/mautic/vendor/symfony/config/Resource/ClassExistenceResource.php - at line 181
@ekke Thank you very much for trying to help me! 