Hi Mark,
thanks for the proposal, which I did not need to implement.
The solution was already posted on tvheadend on this page:
https://tvheadend.org/d/2157-problem-configuring-tvheadend-as-sat-ip-client?r=13477#message-13477
"I found I needed to restart TVHeadend AFTER the TSS had first booted, but it works fine since then. I couldn't get the multiplexes from within TVHeadend's automatic add to work properly and ended up adding one manually myself and after a successful scan (with network discovery turned on) it was able to add the rest (although there was an issue with DVB-S2 Pilots but the option seems to have vanished since then!)"
You have to add one mux manually, then the others are detected. I can see TV streams via VLC, so I think TVH backend is working on my DLINK 320L NAS.
Unfortunately I now have issues with the TVH client on a freshly set up openelec PC. I enabled the client and put the IP address of the backend. It then showed me a new tab for TV on the main screen. However this tab disappeared later although the client was still enabled. When I try to disable the client, the system freezes.