Hi Essekia,
I deleted all the migrated content, deleted the plugin, cleaned the database and reinstalled the plugin one time from repository and one time with a local download. Both times I got the message “plugin headers not valid” after installing. With just the Dummy-Content from you the frontend seems to work, but there ist still an error:
“Notice: Function rest_validate_value_from_schema was called incorrectly. The “type” schema keyword for categories can only be one of the built-in types: array, object, string, number, integer, boolean, and null. Please see Debugging in WordPress for more information. (This message was added in version 5.5.0.) in /homepages/31/d819653946/htdocs/clickandbuilds/Spiolwiese4/wp-includes/functions.php on line 5905“
Your theory that the problem could be generated by migration-failings is possible, because I migrated my Content with an Import-Tool and they all like to make trouble. But after a new installation?
Greetings
Chris