The reason I suspect a DB problem is because I can edit the config file to point to an older DB backup and the error goes away. I have also completly erased all file and started with a fresh copy of VB.
I have already had a lengthy discussion about this on the VB forum. It is here:
http://www.vbulletin.com/forum/showthread.php?t=301844
Where is the modifications thread? Thanks for your help!