Interesting one that has so far stumped the Groupee Support too so throwing this one out to the wider wind...

After upgrading UBB Classic from 6.50 to 6.73 (I backed everything up first and took screengrabs of the settings), the existing list of 4 Forums are no longer visible.

I've checked the existing paths structure (same as before), checked 755/777 permissions, rebuilt all 4 options in the Control Panel and the Forums still aren't showing up through the standard... /cgi-bin/ultimatebb.cgi? route.

Only if I did a direct URL such as the following:
/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=000115;p=0 or

...(changing f for Forum Number; t for Topic Number) will the Forums and Topics show up.

Any ideas guys?

Your help would be truly appreciated.