SQL-DATABASE ERROR

Database error in WoltLab Burning Board (): Link-ID == false, connect failed
mysql error: Unknown MySQL server host 'mysql5.service' (1)
mysql error number: 2005
mysql version: unknown
php version: 4.4.8
Date: 17.07.2018 @ 15:40
Script: /search.php?sid=2d0cf2672aca759906720f105b7d3ed9
Referer: