Based on what you have posted, I am not convinced that you have done anything wrong. I see in your config that you have enabled GRE tunnel keepalives (which is a fairly new feature in IOS). You configuration specifies to send a keepalive every 10 seconds. The debug seems to be showing that every 10 seconds there is an exchange of keepalive messages between the two ends of the tunnel. It looks to me like it is working as expected.If you think that something is not right, can you help explain the symptoms?
You are dealing with hardware with which I have no experience and no real expertise. If it is a problem with hardware or of feature support for that hardware in your particular IOS I will not be much help.
The error message you report when you try to enable server mode and the fact that it works for DVMRP do seem to point to it being an issue with support for the GRE feature on this hardware in the particular version of IOS that you are using. Have you checked for any known bugs in your version of software related to support for GRE?
If this router is covered by a maintenance contract I would suggest that opening a case with the TAC would be the most direct way to get it resolved.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...