Tvheadend

Hello,
I have a Fritzbox cable router (6591 with newest firmware 7.57) in the network. DVB-C is activated in the fritzbox, in TV headend Configuration/DVB Inputs/TV adapters I see all 4 tuners of the fritz box activated with green dots.
In Configuration/DVB Inputs/Networks I have enabled a DVB-C network (create bouquet checked) where 0 muxes are found. If I choose a pre-defined mux I can see the mux amount of this pre-definition.
Also with "Force Scan" TV headend doesn´t start scanning.

At another pc in the same netzwork I can read these files from the fritzbox and watch channels with vlc player via channellist.m3u. So I think the fritzbox is also open for tv headend:
http://192.168.xxx.1:49000/satipdesc.xml
http://192.168.xxx.1/dvb/m3u/channellist.m3u

How can I solve this input problem?