Hi Daniel
In the meantime, the Migros APP Support center called me again.
They got in contact with the customer for further investigation in the problem and think they found the Problem.
The customer has an older ZyXEL WLAN Router with NAT.
The APP apparently uses some kind of ipsec connection. It's not 'standard' SSL they explained to me and the old ZyXEL are known not to support that variant with their NAT implementation.
So the issue is not IPv6 or DNS related.
I wonder what weird IP protocol migros uses :-)
-BenoƮt Panizzon-