I'm not great at code stuff, but it looks like it might be doing the same thing that it was doing here:
https://permies.com/t/179004/Digital-Marketplace-accepting-topics
The path that started us on the right track was to see if you can make a thread in that forum but without the premium tags
Subject: test
body text: test
See if that fixes things.
Then I can press buttons to make it easier for you to edit that post.
And while this is going on, I email the awesome person who makes code go happy.