I have the same problem. It seems that it has got something to do with referer checking. When I run my forum on a different domain (whilst they are the same site), only the main URL seems to work.

Check the shoutbox at the right on my normal ubb.

Working: http://nlforum.merudi.nl/
Not working: http://nlforum.merudi.net/

Same site, with different domains pointing to it. Any clue what is going on here? To me this seems like a bug.