OJS 3.3.x is compatible with PHP 8.0 (though not yet 8.1). However, PHP8 is more stringent with some things than 7.x was, and things that used to result in a warning now result in an error. I think you’re running into a data problem – you can help to identify it by using the SQL query at the following thread:
(Before you start working with the database directly, make sure you have a good backup.)
Regards,
Alec Smecher
Public Knowledge Project Team