What specific model Panasonic TV do you have?

It is a Panasonic TX-P65VT60

Wow. That TV's kind of old. You probably use Netflix via the KPN set top box beacuse your TV doesn't do the apps (anymore). looking at the manual (Download handleiding van Panasonic TX-P65VT60), you can choose to play Dolby form the HDMI-Arc:

I do not understand what you mean by “that is not blue to send Dolby”. Could it be that the TV does not pass through these formats fro  one HDMI to another?

Thanks so far. You are 100% right about the tv and the apps that are not supported anymore on the tv.

The other comment: that was a typo. They say that the KPN box is not able to pass anything else but stereo PCM through the hdmi kabel to the tv.

If it can only pass stereo, how do you expect your Arc to receive and therefore play anything other than stereo?

Does the tv have an internal tuner, and can it receive any channels with programs or films being broadcast in DD 5.1? That will at least check Arc will play that when it receives it. 

It's probably the other way round: the TV cannot pass anything else then stereo to the soundbar. The KPN set top box can certainly do Dolby 5.1 - I used one up until last year wehn I switched proividers.

So either the TV does not pass through 5.1, or there a wrong setting in the KPN box.

Thanks for your contribution. May I ask what setting you used for sound on your KPN box?