I suppose it's possible that Auctiva have different servers for different geographic locations (I don't know the infrastructure layout of course), which I guess might explain why I can get in but you can't - STOP_N_BUY, where are you located?
I would suggest your best bet now is to log a support request (go to Help up at the top), the Auctiva support folk are superb, and I am sure they will be able to help you out.