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: 10.12.2018 @ 07:35
Script: /board.php?boardid=203&sid=91dd86f4ece428667852a8b76564add3
Referer: