In my case that was firewall rules. Was fixed by disabling some of them.

version 6.37.4 on Mikrotik_RB915G Simple (non-regexp) static DNS entries give 2-3 'unknown hostname' errors every day. Often MikroTik answers 'unknown host' immediately (that about static entry!), sometimes it takes up to 20 sec and the answer may be positive or negative. External DNS names are also...