according to this post https://shop.tbsdtv.com/forum/viewtopic.php?f=2&t=193
I had been able to get more details about the problem. It seems that the card lost's the signal after some time.
In detail:
I connected a stand a lone receiver to the cable and had a good signal with all channel I need.
Then I connected the same cable to the tbs 6981 and started channel search using tbsviewer. It found some channels and seem to work.
After this I changed the preffered card, using the options, and started the channel search again. It found some channges, but after maybe 20 channels, there was no more signal.
When I disconnect the cable, connect it back to the stand alone receiver, I get a good signal as before.
Reconnecting the cable to the tbs 6891 starts the same procedure as described above.
Either the card is broken or I have a power supply problem in my htpc?
Any ideas?