Login, logout & identity.php problems - urgent help needed

[outlet.ee]

I received a complaint from a customer who has trouble with login (authentication failed). I can log in but identity.php gives me error (see attatched) and I can not log out, it redirects me to a blank page now.

Moved to a new server recently and upgraded from 1.2.5 to 1.3.1, I think it has something to do with it.

Is there anybody who could tell what's wrong by the message attatched.

idendity.php_error_message.txt

[rocky]

I think the problem may be caused because classes/Customer.php is looking for the isDni() in classes/Validate.php and cannot find it. Make sure that classes/Validate.php has the isDni() function and the file has been uploaded properly. If you try to use the Validate.php from v1.2.5 with the Customer.php from v1.3, you will get this error.