I finally managed it but not with the CA in tvheadend. I had to use oscam.
The key is definitely the correct one (since it works in oscam).
The available options for a CA in tvheadend now are
* Code Word Client (newcamd)
* CCcam
* CAPMT (Linux Network DVBAPI)
* CSA CBC Constant Code Word
* DES NCB Constant Code Word
* AES ECB Constant Code Word
* AES128 ECB Constant Code Word
I used the "DES NCB Constand Code Word", maybe I should try another one?
Oh, well, with oscam it works, so problem solved.