Well now I am having a similar problem and currently cannot take ANY orders because of the problem. Have had to shut down my store because of it.
http://www.vacsew.com/home.php?shopkey=904
Try placing any order. When you get to the checkout page,
Click on "New customer? Click here to register"
Fill out the form. Hit submit. Notice that it keeps bringing you back to enter the username. Try entering a username, still brings you back.
The weird part is that it was working just fine until yesterday. No changes or anything.