Thank you!
That was funny, because I had no version of youtube-dl installed at all!
I didn't know, that this step was required.
Now it streams both of them just fine!
One last question ... if I may?
The video seems to buffer 4 min ahead, which I'd like to reduce to 10 sec.
But more importantly: it just keeps the last 4 min in its memory, so if I want to watch the video again, after its finished ... it repeats the whole download again. So in short: can I increase the memorized chunk to the full length, just like the native youtube-web-player does?