The fix is quite simple: in login.processor.php...
...line 55 (instead of html_entities):
$username = $modx->db->escape($_POST['username']);
$givenPassword = $modx->db->escape($_POST['password']);
...line 67 (get rid off "REGEXP"):
$sql = "SELECT $dbase.".$table_prefix."manager_users.*, $dbase.".$table_prefix."user_attributes.* FROM $dbase.".$table_prefix."manager_users, $dbase.".$table_prefix."user_attributes WHERE BINARY $dbase.".$table_prefix."manager_users.username = '".$username."' and $dbase.".$table_prefix."user_attributes.internalKey=$dbase.".$table_prefix."manager_users.id;";