I have the same issue, but it arose after I messed around with the iOS app. Now it immediately hits me with expired credentials whenever I sign in on desktop…
I am unable to sign in after clean installing my MacBook Pro to High Sierra. I can log in to plex.tv but anything else, it just fails and the same message appears again and again.
A response from Plex: https://forums.plex.tv/discussion/292803/has-plex-been-hacked-again-oct-2017
It’s a server that keeps keeling over, apparently.
Same Thing Was Going On With Me! I Went Back To (DEVICE STORE) And Reinstalled The App!
Before login----Than Had To Login, For The App To Be Installed, — (LIKE AN UP DATE)—Work For Me.