[ManiacKill]

After month of seamless use, yesterday suddenly the YT plug-in has stopped starting any video. With any videos it says (upper right corner): YouTube / No streams found sometimes with a splash screen: Playback failed: One or more items failed to play. Check the log for more information about this messege. OK
I can see the channels, videos etc., even the index pictures of the videos. After a clean reinstall the problem remained the same: none of the videos can be started.
I use a Raspberry Pi 3, Libreelec (9.0.1/18.1 but upgraded to 9.1.002/18.3 in hope it will solve the problem, same result), tried three or four different YT plug-in version, 6.5.1 and 6.5.2-beta1 included.
Here is my debug log: https://pastebin.com/eGiSQF3R

Thanks in advance for any kind of imformation or idea, what am I try next!

Same here, and I get the same errors as you in debug log.  Everything plays OK when I'm signed out, and live streams still play when I'm signed in.  I'm in UK, in case it's something to do with location.  Running v6.5.1 of the add-on, OSMC Matrix beta 18.8-82 on RPi3B+ (I know, beta OSMC, but the Youtube videos broke separately from the beta updates).

Have tried changing the stream selection options in the add-in (turning off MPEG-DASH, setting video quality to always ask, turning inputstream-adaptive on/off), it doesn't seem to make any difference.  And as said, signing-out of Youtube seems to make all videos play as expected. 

Is there already a solution to the problem?