ddclient is working.
I checked it with:
sudo ddclient -foreground -file
/home/florian/repo/myLinux/data/nuc/ddclient.conf

The cache file gets updated, sudo cat  /var/cache/ddclient/ddclient.cache
## .ddclient-real-3.9.1
## last updated at Fri Dec 24 11:08:10 2021 (1640340490)
atime=0,backupmx=0,custom=0,host=nuc.pinkgorilla.org,ip=85.125.195.170,mtime=1640340334,mx=,script=/nic/update,static=0,status=,warned-min-error-interval=0,warned-min-interval=0,wildcard=0,wtime=30
nuc.pinkgorilla.org

I can also run ddclient without sudo, but this then does not update
ddclient.cache file. But it does its job.

As a service it is not working though. This is my service definition.
service-ddclient-nuc is used in the operating system definiton.
(define-public service-ddclient-nuc
(service ddclient-service-type
(ddclient-configuration
(secret-file "/home/florian/repo/myLinux/data/nuc/ddclient.conf")
)))


in var/log/messages I get this error(I just pasted 3 errors; it repeats
every 5 minutes).
Dec 24 10:15:34 localhost .ddclient-real[10715]: WARNING:  found neither
ipv4 nor ipv6 address
Dec 24 10:20:34 localhost .ddclient-real[10715]: WARNING:  found neither
ipv4 nor ipv6 address
Dec 24 10:25:34 localhost .ddclient-real[10715]: WARNING:  found neither
ipv4 nor ipv6 address

Perhaps the ddclient service needs to wait until network is available?