A few people are having issues - when they go to pay via PayPal or Credit Card they get looped back to their shopping cart. I've read a bit about this and it seems most of the errors originate from the UK. This isn't happening to a lot of people but I've been trying to get to the bottom of this. PP has been having some issues lately.
I assume you checked with them that they have cookies enabled? (at brickforge.com and paypal.com) Cookies disabled tends to cause looping problems on some sites.