After update from 2.1.1 to 2.1.3. previously registered users with cyrillic symbols in their usernames can't login to forum. Error message "The user name contains invalid characters" appears... :(
How can I fix the problem?

Some correctives: registered user with cyrillic nikname can login to forum, but he can't create new topic or answer to other messages - error message appears.

This check is only performed when registering. New user names are checked for some control characters. This is done by the function contains_special_characters() (includes/functions.inc.php). What happends if you change it like this?

function contains_special_characters($string)
{
return false;
}

This code allows to bypass check, and "cyrillic user" can publish messages.