Hi, thank you for your help.
I did check the status of the Omnia with the “ifstatus wan6” command. I can see that a \62 prefix has been assigned to the wan6 interface. All computers behind the Omnia receive ipv6 adresses. However, I can not reach any ipv6 host on the internet - I always get the error “network unreachable”.
The other problem (fritzbox configuration page not accessible) just vanished into thin air after a reboot of all devices.