Thank you, now it works!
When I edited the channels I found that they were set to DVB-T instead of DVB-T2. Maybe this was the problem.
Anyway I found 19 channels.
But now every channels appears to freeze every second and the audio gets out of sync. Then after some 30 seconds kodi starts to buffer again. Sometimes the audio disappears completely and doesn't come back and also playback stopped completely a few times. The temperature gets very high also I have opened the case and use heat sinks. It seems as if the RPi 3 is not fast enough for the h.265 decoding.
Does anybody not have this problem?