Excellent. That Failed to insert record to database. is exactly what we’re looking for. The new version of the sidebar that we added in 5.26 store more information about the configuration of the sidebar in the database, so it looks like it’s failing to actually save that new information in the database.
Looking at the Error 1267: Illegal mix of collations (utf8mb4_0900_ai_ci,IMPLICIT) and (utf8mb4_general_ci,IMPLICIT) for operation '=', MySQL is having issues working on two database tables at once, likely the existing tables and the ones for the new sidebar. Have you recently upgraded to MySQL 8? According to this, MySQL 8.0.1 is what changed the default collation.
To fix the issue, you’ll need to change the collation of the database tables to match. I’d recommend changing the older tables to match the new ones since any future database tables that are created will also match the new ones. This StackOverflow answer should have information on how to do that.
Also, you can change the log levels back to whatever you had them set to before if you want since the DEBUG level is a lot noisier.