Default [Q] VPN to a Fritz!Box doesn't work on Omnirom

Connecting my N1 via VPN (IPSec Xauth PSK) to a Fritz!Box 7390 doesn't work - it tries to connect but doesn't succeed, the VPN is then shown as disconnected.

I followed these steps:
(I am not yet allowed to post links so please copy and paste. Sorry, it is German only, but if you scroll down to point 3 you can see a screenshot which shows the settings.)

Omnirom Version is 4.4.2-20140510-n1-nightly

The same settings work with a Nexus 7 3G 2012 / Omnirom Version 4.4.2-20140510-tilapia-nightly.

Does anyone have the same problem? Is there a solution for it?
Any help is appreciated.