@jacadcaps
Ok, now it works fine with Wayfarer,
The only problem is that the first video you choose to see you will have to press for example "space bar" from the PowerBook but from that moment on any video searched with the youtube-mobile app will start without problems.

Without using Wayfarer or the PowerBook keyboard it will basically work like a remote control etc.

While with OWB the video starts immediately.

That's why with Wayfarer the black screen remained, I had to start the first video with the "space bar".

Thanks for your suggestions.

here a short video with Wayfarer:
https://we.tl/t-OJjDvQ2Wew