The Plex client included in the standard Kodi repository hasn't been updated for quite a while, and I wonder if it ever will.
I too couldn't get it to work without issues under LibreELEC 11.
I finally switched to a newer client using the following repository: https://pannal.github.io/dontpanickodi/
See link below for more info.
PM4K / PlexMod for Kodi (18, 19, 20, 21) - Plex Players / Desktop Players - Plex Forum