This happens about once a month on the Nexus Player. I have stopped automatic updates because I though I was when I got a new version it happened but I now can say for sure it has nothing to do with updates.
Every time it happens the app get the sign up screen and I need to go to plex.tv/link and hook it up again.Since my wife doesn’t have a clue the entertainment at home gets halted until I’m home. It’s denial of service!
This also happens with the android/iphone app but I need to login with username and the horribly long password lastpass has choosen for it that requires changing back and forth between 2 symbolskeyboard and letters 8 times. It’s quite annoying!
I have a lifetime Plex Pass subscription. Right now it seems Kodi was less messy!