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.