I have posted this in the infopop's community, then I just found this site.

There is a bug in the ubbt.inc.php. When a user logged in, the code called addslashes() twice for the username. For some users using special charaters, it would show extra slash in the online list.

// Update the who's online screen
$Username = addslashes($user['U_Username']);
$What = getenv ("SCRIPT_NAME");
if(strstr($What, 'admin')) {
$What = "all_admin";
}
else {
$script[0] = "";
preg_match ("/(.*)\/(.*).php/",$What,$script);
$What = $script[2];
}
$Last = $this -> get_date();
$What = addslashes($What);
$Username = addslashes($Username); <<-- remove this line

Remove the line, the user online list will not show addtion slash.

Fixed in v5.5 []https://www.ubbdev.com/threads/php/images/icons/smile.gif[/]