Thanks for the effort, much appreciated.
Honestly that bears the question where you got the crt and key files for/from the fritz.box?

When i follow fritz manual and create vpn users i end up getting a page with manual iOS/Android insructions.
Those credentials then work for me from my linux desktop and once when i set up vpnc from cli on sfos manually.
But i have no clue where to get the keys and certs to put into vpn gui config