I was stuck at the same "Unfortunately we are unable to complete registration" message for my wife's account. I kept hacking away trying different things. Finally I made three changes at once and it went through.
- I changed the username that I had selected to be the email address associated with the account
- I put something into line 2 of the address field, which was blank before (even though there is no asterisk in the field)
- I changed Mrs to Ms in the top name field.
Not sure which change made the difference but it worked.