When I try to enter the theme manager I get this error, I have repeatedly loaded the thema, deleted the thema, now even the default one does not work
It is best if you activate the error debugger in your config file to get a detailed error response: https://docs.shopware.com/en/shopware-5-en/tutorials-and-faq/debugging-error-messages-in-shopware#tweaks-for-the-config-php
This issue in the theme manager is often caused if you have a child theme that misses a parent theme. Check your theme/frontend/ directory and delete any child themes without an existing parent theme.