My guess is in ubbthreads.php change

PHP Code
$result = ppmysql_query("UPDATE {$Globals['dprefix']}USERS_DATA SET USER_LAST_VISIT_TIME=$date WHERE USER_ID={$User['userid']}",$db_link);  

to

PHP Code
$result = ppmysql_query("UPDATE {$Globals['dprefix']}USER_DATA SET USER_LAST_VISIT_TIME=$date WHERE USER_ID={$User['userid']}",$db_link);