This is mostly a report to Peter (the admin) so he can fix, but in the meantime it might also help anyone else encountering it:
When I create a new thread (like this one) and uncheck the "Subscribe to this thread" checkbox, I get an error page after submission, and the thread doesn't get created. (This bug has existed for years, but I finally figured out the cause.) When I leave the checkbox checked (its default state), creating the thread works fine (and I can unsubscribe on the results page).
[ Edit: Edited on 1 Mar 2023, 19:23 GMT by Sander ]