Seems there is an issue with LibreTube. Message crops up:
"initial player response is not valid". There is a ticket raised already.
https://github.com/libre-tube/LibreTube/issues/4134
This issue could be in response to YT's crackdown on the unauthorized use of their service. The Invidious servers were also targeted in the last month or so.
In the meantime, hop over to NewPipe and download their app if you haven't done so already on F-Droid, Droidify etc.
#whackamole