Tvheadend

I am facing some trouble with my current setup.

I have a unicable LNB which connects with one cable to a TRIAX TSS 400 Sat-IP convertor with 4 tuners available. (it is NOT the mkII-version!)

So, I should have 4 tuners available. The 4 Frequencies have been configured on the TSS400-Config-Website.

As backends (or frontends?) I am using ....

... 1 Panasonic VIERA-Tv which is using IPTV
... a tvheadend-setup which seems to be faulty.

1. The VIERA is working fine on all stations, so I think my LNB and also the TSS400 is working fine.
2. My tvheadend-setup is working sometimes, but sometimes it is not. The main problem is "RTSP setup error -5 (6-503)

Sometimes, it seems that tvheadend connect does not know that one of the tuners is already occupied from my VIERA.
How can I make sure, tvheadend uses a free tuner?
How can I limit tvheadend to use only 3 of the 4 tuners (leave on reserved for the VIERA)
How to setup the DVB-inputs the right way?
At the moment it says "Auto" and I have no idea if that is okay!