That's the "correct" way, if the addon is the the problem, change:
$title = !empty($user['last_login']) ? 'title="' . standardTime($user['last_login']) . '" ' : '';
to:
$title = !empty($user['last_login']) ? 'title="' . strip_tags(standardTime($user['last_login'])) . '" ' : '';
in OnlineToday.class.php.