Hi !
As far as I know, currently (ASA OS Version 9.9(1)) ASA 5506-x does not support IPV6 over PPP / IPV6CP / RFC 5072. Therefore it is currently not possible to use IPv6 with German Telekom VDSL.
A fixed ipv4/ipv6 address for German Telekom VDSL does not help.
It is possible to use IPv6 buliding a router cascade.
Question: Is there a statement from Cisco like "We know of this problem, we are working on it, wait for a future ASA OS Version" or is there a statement "We are currently not working to solve this problem?"