theming – Why do I get an error after adding a .theme file?

After creating (e.g empty) .theme file in my template folder, I get the following error message:

The website encountered an unexpected error. Please try again later.RuntimeException: Failed to start the session because headers have already been sent by “/homepages/d825952918/htdocs/themes/universaldenker/universaldenker.theme” at line 1. in SymfonyComponentHttpFoundationSessionStorageNativeSessionStorage->start() (line 145 of vendor/symfony/http-foundation/Session/Storage/NativeSessionStorage.php).

If I delete the .theme file, everything works again. I think it has something to do with opcache, because the path “/homepages/d825952918/htdocs/themes/universaldenker/universaldenker.theme” comes from the .opcache folder.

How can I tell Drupal not to use it and clear all the cached files?