What about starting a Linux Live Image (anyone you're familiar with) and editing manually your interfaces config file ti IP4? That's what I would try...
YES!!! Worked. I guess my luck is, that I have a second partition, because I could'n access the primary partition through the live disk. The second one was writeable as root.
I changed the entry in the "interface" file from:
iface eth0 inet6 dhcp
to:
iface eth0 inet6 auto
now the system is up again and my server has a valid IPv6.
One more hint:
after login OMV detects a change and ask you, if you want to apply the new settings, but you didn't change anything...
Discard this request!!!
Go to your network settings and apply here once more the "Auto" point for IPv6 (or disable it again).
Thank you very much! Hope it helps someone, too.
