UBB.threads PHP Forum Software Community Forums UBB.threads Version 7/8 Support Submit not working / hanging

Hi,
suddenly my users cannot longer post (UBB Threads 7.3, Windows Server, PHP 5.2.2, Firefox and IE tested).

When I click on the submit button, the session hangs for minutes. I see no activity on the server, the file system (using FileMon) or in MySQL.

Preview works. Deleting a topic also works. Only post fails. I'm not sure where else too look.