Update:
I found out where the ActivateTunnelAndPDR error is coming from:
in smf/context/upf.go the function IP(…) starting in line 105 is causing the problem. (smf/upf.go at 04c01ec5dd4e0e69342e9047a1d37442d88d9904 · free5gc/smf · GitHub)
Only PDUSessionTypeIPv4 and PDUSessionTypeIPv6 are supported here, however, in my case it was PDUSessionTypeIPv4IPv6. I just added an additional if statement catching this case and returning the IPv4 address.
best,
Ralf