Can't finalize order

-- url of the page with the problem -- : metebutiken.se
-- HikaShop version -- : 2.5.0
-- Joomla version -- : 3.4.3
-- PHP version -- : 5.4.42-servage17
-- Browser(s) name and version -- : N/A
-- Error-message(debug-mod must be tuned on) -- : Login or register new account

Help!
I have problems with two (at least) of my clients web shops.
The problem is that you can't register a new user all of a sudden.
It is only possiblr to place an order as a registered customer.
And sometimes not even possible that way.
When you press the submit order buton in the end of the checkout process you get an error message like this:
"Login or register new account"
Please I need help with this, what can be wroong with the registration process?

Best regards
Micke