My guess is in ubbthreads.php change
$result = ppmysql_query("UPDATE {$Globals['dprefix']}USERS_DATA SET USER_LAST_VISIT_TIME=$date WHERE USER_ID={$User['userid']}",$db_link);
to
$result = ppmysql_query("UPDATE {$Globals['dprefix']}USER_DATA SET USER_LAST_VISIT_TIME=$date WHERE USER_ID={$User['userid']}",$db_link);