I gotta admit, the error message you now get when creating a new thread is a little frustrating:

"You must select a thread prefix."

Of course, I went ahead and typed in a prefix ("[M5]"), but that apparently didn't count. I finally found this thread, and added myself to the Developers group.

I think a better error message would be,

"You must be in the Developers group to create a new thread."

That would get you a lot closer to figuring out why you can't post.