Okay. That's pretty much what I expected.
Sadly, that's not good news.
The problem seems to lie in the forum section permissions assigned to each member. Moderators can post, but others can't. That should be easy to fix, but the problem is that it's already fixed, or at least set to the correct configurations. The simplest solution would be to just get rid of the Hatchery.
I have an idea and will test it. It it works, then it's still no help, but at least I'll have it pinned down.