"Subquery returns more than 1 row" error on quick registration

Hello

While trying to set quick registration on our site we get "Subquery returns more than 1 row" on a blank page.

What we need to do is...

Have the users able to register filling just a few fields at the beginning, and then when loged in, to complete their profile fields. !!! (Some of the remaining fields re mandatory) !!!

Can you please advise about the "Subquery returns more than 1 row" error?

Thank you in advance.