Thanks Connor,
I contacted the ISP and they switched my account from dual (IPv4+IPv6) to IPv4 only.
We also restarted the Rooter.
Both actions without success.
The ISP is providing static IP addresses to business clients only.
Beside that, Roon ARC worked very well before the Innuos Zen Mk3 (my roon core) got updated. And I cannot imaging that innuos had the need of fixed IP addresses in mind when providing the update.
Any other possiblities we could try?
BR
Peter