You've posted your issue in the "User Registration Solution Pack"-Forum, so that's why I asked if you were using Data Bridge, as there might be a difference.
However, I've never used the "User Reg. Sol. Pack". So, there could be a different approach for this WA product.
I've found this link:
Maybe this helps you a bit.
P.S.: With Data Bridge I've never run into such an issue, except when checking the "log me in automatically" checkbox. Of course that wouldn't be a problem with the extension, but my own error, just forgetting about the cookies to log in automatically...