Ok, to get more info on what's going on, you can put libicd-network-3g-ipv6 into debug mode:

[run these commands as root, it'll disconnect you]
initctl stop icd2
/usr/sbin/icd2

This will print out the connection messages to your terminal.

If it's a matter of not having ipv6 support, it'll say:
ifconfig: socket: Address family not supported by protocol

If it's something else, it should say something like:
[X] failed (com.nokia.[Y]): [Z]

That error message should hint to what's going wrong.

After you're done with debugging mode, you can turn it back off with:

[this will disconnect you again]
hit control-c and you'll get a prompt back
initctl start icd2