AVM FritzBox: dns resolving fails with Warning: Client COOKIE mismatch

I have a strange dns problem. AVM Support seems to be core-disabled. So I try to ask here.
I have exactly one host in my lan that can’t be resolved via fritzboxs dns resolver.
Workaround is obviously using a static hosts entry, but I want to figure out what the problem is.

I tried to change the dhcp range in order to reset dns – didnt work.
Windozish reboot solution didn’t work either.

Each host of the lan has the same problem resolving “nas”
Due to that I state that the fritzbox is the root of the evil

Stranger: The fritzbox-own nas was enabled (dont know why). But after disabling it, it worked for a short period of time.

# Works:
dig otherhost @fritz.box +short

# Workn't
dig nas @fritz.box +short 
;; Warning: Client COOKIE mismatch

# long version
;; Warning: Client COOKIE mismatch

; <<>> DiG 9.16.6 <<>> nas @fritz.box
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 50705
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 614114a32d8b03583e78bdc05f5b4b692214951638920c44 (bad)
;nas.                           IN      A

.                       10753   IN      SOA     a.root-servers.net. nstld.verisign-grs.com. 2020091100 1800 900 604800 86400

;; Query time: 0 msec
;; WHEN: Fr Sep 11 12:04:09 CEST 2020
;; MSG SIZE  rcvd: 135