If on astaro is Prefix Advertisement enabled and configured, then the Windows DHCP V6 Server disabled it byself. --> this effect is only if antoher DHCP Service is running on the same LAN.

My Question: Why start UTM9 the dhcpd6 and listen on eth0 (internal LAN) if Prefix Advertisment is enabled ?

The deamon radvd is the only one, we needed! if UTM9 is not the primary DHCP for IPV6.