When peoples create account witn non-ASCII symbols (for example russian symbols) username, confirmation e-mail contain bad link (link only for ASCII symbols and plain text username).
For example, username: ?????
Link: http://www.cdp.tsure.ru/moodle/login/confirm.php?data=KnIypNG1kDuK9nJ/?????
Only http://www.cdp.tsure.ru/moodle/login/confirm.php?data=KnIypNG1kDuK9nJ/ - link and "?????" - plain text. Peoples can't confirm account and it confused theirs...

Solution:
in moodlelib.php in function send_confirmation_email use urlencode for $user->username
$data->link = $CFG->wwwroot .'/login/confirm.php?data='. $user->secret .'/'. urlencode($user->username);
instead of
$data->link = $CFG->wwwroot .'/login/confirm.php?data='. $user->secret .'/'. $user->username;