Do you have multiple AVM products in a Mesh configuration? There is a known bug (this usually also involves switches between the Mesh devices) where some devices aren't reachable anymore in your network. One typically doesn't notice this issue, however, with the DNS server you always notice it because it basically means the network is down.
It is a confirmed issue by AVM support, they're working on a fix but (apparently) don't yet know how to solve the issue. The best advise they have right now is removing any "indirect cabling" between the devices and to connect all devices directly to the Mesh Master.
There s a pretty long discussion about this in German (if it is the same problem):