HGold reg code--25 characters plus four dashes--doesn't fit in the input box in-game. The very last character isn't accepted. This is in the latest HGold, purchased today 2011-10-15, so maybe a small bug introduced in the latest patch four days ago.
It's an easy fix, just delete the dashes, enter the 25 characters and the reg goes thru. But it might stymie a few customers, so fyi.