Suddenly IPv6 in initramfs but no SSH login

I have already asked this question here:
http://askubuntu.com/questions/594726/initramfs-suddenly-speaks-ipv6-but-no-ssh-login
but haven't gotten an answer yet.

I have been running a configuration with an encrypted root partition and network unlock via Dropbear in initramfs for some time on my Utopic Xubuntu Mediacenter. Since I am actively using IPv6 to access devices in the network from outside, it has always been a pity that initramfs-tools didn't support IPv6.

Now, without me knowing of any major update, the machine is suddenly reachable via IPv6 before the root partition is accessible, BUT neither my rsa key from /etc/initramfs-tools/root/.ssh/authorized_keys nor my password as a user are accepted (latter makes sense of course, since /etc/shadow is not decrypted yet).

dmesg says NOTHING about what's going on, this is all that is logged from boot, over two unsuccessful login attempts via ssh until decryption locally on the keyboard:

[ 3.059032] input: Logitech Unifying Device. Wireless PID:2010 as /devices/pci0000:00/0000:00:12.0/usb3/3-4/3-4:1.2/0003:046D:C52B.0007/0003:046D:C52B.0009/input/input9
[ 3.059179] logitech-djdevice 0003:046D:C52B.0009: input,hidraw6: USB HID v1.11 Keyboard [Logitech Unifying Device. Wireless PID:2010] on usb-0000:00:12.0-4:2
[ 3.225536] Switched to clocksource tsc
[ 3.910848] r8169 0000:02:00.0 eth0: link down
[ 3.910871] r8169 0000:02:00.0 eth0: link down
[ 3.910920] IPv6: ADDRCONF(NETDEV_UP): eth0: link is not ready
[ 6.458419] r8169 0000:02:00.0 eth0: link up
[ 6.458436] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[ 46.401849] random: nonblocking pool is initialized
[ 66.931909] raid6: sse2x1 3291 MB/s
[ 66.999840] raid6: sse2x2 3691 MB/s
[ 67.067780] raid6: sse2x4 4607 MB/s
[ 67.067782] raid6: using algorithm sse2x4 (4607 MB/s)
[ 67.067784] raid6: using ssse3x2 recovery algorithm
[ 67.069904] xor: automatically using best checksumming function:
[ 67.107736] avx : 6114.000 MB/sec
[ 67.130684] Btrfs loaded
[ 67.151180] BTRFS: device fsid 47b1efee-d626-41dc-97e3-07205d612763 devid 1 transid 222612 /dev/mapper/sda2_crypt
[ 67.158124] BTRFS info (device dm-0): disk space caching is enabled
[ 67.199031] BTRFS: detected SSD devices, enabling SSD mode
[ 67.369117] init: plymouth-upstart-bridge main process (339) killed by TERM signal

Can anybody tell my what is going on here?

    * Why is IPv6 suddenly possible, though there was no major update and it is documented everywhere that it is not supported yet?
    * How do I get authentication running again?
    * Where can I find additional logs?
    * Should I maybe even worry that there is some strange new network capability with unknown functionality?