The theme issue seems to be a red herring. I have just loaded it on to my dev XOOPS instance and it works as expected. You can log in to the dev site, which is on the same server in a folder at the same level as the live site and it all works fine. However the live site still fails.
Also just realised that this is a hosted server (nor is it Win2K), not my own, so I am just checking with them now.
The live site is athttp://partners.insightexec.com/
The dev site is athttp://partners.insightexec.com/dev/
Cheers,
GadgetMn