White snow agains the black ice ...

Normally, to configure IPv6 (and v4) forwarding on a Linux system you should edit the /etc/sysctl.conf file and change the lines like this:

# Uncomment the next line to enable packet forwarding for IPv4
net.ipv4.ip_forward=1

# Uncomment the next line to enable packet forwarding for IPv6
net.ipv6.ip_forward=1

This way your kernel will load the forwarding parameters on next reboot.
To activate the feature without rebooting do:

# echo 1 > /proc/sys/net/ipv4/conf/all/forwarding 
# echo 1 > /proc/sys/net/ipv6/conf/all/forwarding

But suddenly you get the following error with ipv6:

# echo 1 > /proc/sys/net/ipv6/conf/all/forwarding 
-bash: /proc/sys/net/ipv6/conf/all/forwarding: Permission denied

In my setup I run Linux as guest in OpenVZ. It seemed my OpenVZ wasn't fully configured to allow IPv6. (I run it trough Proxmox). To enable full IPv6 support I edited the /etc/vz/vz.conf file and set IPV6="yes".

This still didn't solve the problem, even after rebooting the server completely.
Currently I couldn't find a way to get it working correctly, so I created a fully virtualized machine with bridged networking.
I hope to find a way to get ipv6 forwarding working correctly in OpenVZ soon, a post with the solution will follow then.