We have just updated to latest version of UBB. threads MySQL and one of us (very active user - moderator) have a problem with posting. He gets such strange message
The host from which you are accessing the board is not recognized as a valid host. This is more than likely related to a firewall issue that is blocking the referer variable. Check your firewall settings and try again.
He hasn't had such problems before when we have UBB.classic forum.

Warm regards
Antoni Ozynski

It boils down to either:

1. Virus Scanner
2. Internet Security Suite
3. Firewall

One of the above is blocking the referrer variable from populating from their machine.