To sort out ipv6 issues, I tried to connect inside my local lan. The ipsec ports are also reachable not only from the outer internet. But also with this setup, no connection was possible.
Then I tried an old Android phone. The VPN connection worked almost on the first try. So, the FritzBox is setup correct.
I will stop trying to fix the issue in my SFOS version and will now update to the latest firmware first. Hopefully, after that the VPN connection will also work on my side…