mpjgregoire, 
Finally, I have #Plex working again on #Kodi to view movies — it hadn't been working for two months or so.
The solution was to install #PlexMod for Kodi, https://forums.plex.tv/t/pm4k-plexmod-for-kodi-18-19-20-21/481208
I read about it at https://discourse.osmc.tv/t/plex-not-loading/97080 . Darwindesign says the official Plex add-on is broken, and #PM4K is actively maintained.