IPv6 prefix delegation to Fritz!Box no answer from pfSense

[junicast]

Hi,

pfsense 2.3.2-RELEASE-p1
FritzOS 06.80

I have a pfSense system running on a VDSL link with a static IPv6 configuration + a static routed 2001:1111:4444::/48.
Now I would like to run a Fritzbox behind that pfSense while requesting a prefix through prefix delegation.

I set up everything like I think it's right, but as it seems the FB asks for a prefix, but the pfSense just doesn't answer. What am I doing wrong. Here come the details.

pfSense:
downstream interface IP: 2001:1111:4444:1c::/64
Enable DHCPv6 server on interface VOIP
DHCPv6 Server range from: 2001:1111:4444:1c::f
DHCPv6 Server range to: 2001:1111:4444:1c::ffff
DHCPv6 Server PD range from: 2001:1111:4444:f000::
DHCPv6 Server PD range to: 2001:1111:4444:ff00::
DHCPv6 Server PD size: 60
Router Advertisement is set to Managed with DNS Settings set to "Use same as DHCPv6 server"

FB (translated from German):
use native IPv6
global address exclusively via DHCPv6
As a matter of fact I tried every possible setting in the FB. It doesn't change.

Then I sniffed on pfsense at this VOIP interface with wireshark to see what happens.
I see a DHCPv6 Solicitation originating from the FB headed to ff02::1:2.

Please help!