I've experienced this problem as well, Rich. Every time it happened it was an issue of the browser I was using not liking MS's UBB software. Usually I could solve the problem by logging out and logging back in again.
I think these problems are going to be addressed with the UBB people when the time and funds are available.
“Life’s journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy ____…! What a ride!’” ~Hunter S. Thompson