Sorry to hear that. There is the option of removing all your Plex library metadata including the watch statuses and playlists: https://support.plex.tv/articles/201198426-restart-plex-media-server-setup-from-scratch/
You will have to re-setup your Plex libraries and re-download fresh metadata.
You could continue using PlexKodiConnect - BETA version 2.7.5 should fix the playlist issue. This cleaning up is hence a one time thing
Edit - make sure every Kodi instance is not running when doing this. Additionally to the steps you need to do that I linked above, you need to delete the playlists subfolder in the Kodi userdata folder for every Kodi. Otherwise, playlists get synced back
