I have the same problem with Kodi 17.4 on Mi Box. If I put the box on sleep for a long time I get the three dots, and they're preventing me to quit kodi or do anything.
The only workaround I found is to auto-start the plugin when kodi launches, and when I my watching session is over, quit Plex plugin (and optionally Kodi too) before sending the device to sleep.
I have the same issue, if I don't use my Android box for a few days, it wakes up but Plex appears to crash. Only way to recover is to pull the plug on my box then restart.
After reading through these threads, I guess I should count my self lucky that at least my setup seems to work (fingers crossed) given the apparent lack of dev for this app and problems other people are having.
I'd rather just use the Plex for Android app, and not go through KODI, but hardly anything plays (4k, x265).....
Shame because I generally think Plex rocks, when it works.....