Thank you for the quick answer. I also found out last night (about 3 o'clock
. The problem was that the field was defined as "not NULL" in the database. I allowed "NULL" and the problem was solved. Then I wrote to Kunena. Today I also got the answer from Kunena recommending this.
Funny detail on the side: the problem was solved by Kunena months ago. With the current version, the bug fix was apparently gone again. That's why the error only occurred when I updated to the new version.