I have Debian 9.13, but I did resolve the problem by completely restarting my container, re-installing the .deb file, and then restarting plexmediaserver itself:
(as root)
dpkg -i plexmediaserver_1.31.3.6868-28fc46b27_amd64.deb
reboot
service plexmediaserver restart
This worked fine for about a week, but then all my libraries disappeared suddenly.
