bbPress.org

What version bbPress did you install? Maybe it’s a bug changing from a category and then back not working.

I have an Alpha 2 installation, and I created a forum, made it a category, then added some sub-categories. Then, I unchecked the “Forum is category” on the top forum, and it became a regular forum, and I was able to post in it. At that point it was just hierarchical, with a top level forum and three sub-forums, not a parent category that would not allow posts. I think this is exactly how the “Forum is Category” functionality is supposed to work.