Re-installing 1.18 works fine and starts back up. Results of that install below.
sudo dpkg -i plexmediaserver_1.18.6.2368-97add474d_amd64.deb
dpkg: warning: downgrading plexmediaserver from 1.19.1.2589-2de7f3266 to 1.18.6.2368-97add474d
(Reading database ... 119229 files and directories currently installed.)
Preparing to unpack plexmediaserver_1.18.6.2368-97add474d_amd64.deb ...
stop: Unknown instance:
Plex Media Server is not running.
PlexMediaServer install: Pre-installation Validation.
PlexMediaServer install: Pre-installation Validation complete.
Unpacking plexmediaserver (1.18.6.2368-97add474d) over (1.19.1.2589-2de7f3266) ...
Setting up plexmediaserver (1.18.6.2368-97add474d) ...
PlexMediaServer install:
PlexMediaServer install: Now installing based on:
PlexMediaServer install: Process Control: Init
PlexMediaServer install: Plex User: plex
PlexMediaServer install: Plex Group: plex
PlexMediaServer install: Video Group: video
PlexMediaServer install: Metadata Dir: /var/lib/plexmediaserver/Library/Application Support
PlexMediaServer install: Temp Directory: /tmp
PlexMediaServer install: Lang Encoding: en_US.UTF-8
PlexMediaServer install: Config file used: /etc/default/plexmediaserver
PlexMediaServer install: HW transcoding: Found
PlexMediaServer install:
PlexMediaServer install: Completing final configuration.
System start/stop links for /etc/init.d/plexmediaserver already exist.
PlexMediaServer install: PlexMediaServer-1.18.6.2368-97add474d - Installation successful.
Processing triggers for libc-bin (2.19-0ubuntu6.15) ...
Processing triggers for mime-support (3.54ubuntu1.1) ...
cat /tmp/plexinstaller.log
# Plex Media Server installation configuration info: Sat Apr 4 14:11:42 AEDT 2020
Init=1
Systemd=0
LinuxContainer=0
NewInstall=0
HaveOverride=1
OverrideFile="/etc/default/plexmediaserver"
PlexUser="plex"
PlexGroup="plex"
VideoGroup="video"
AppSuppDir="/var/lib/plexmediaserver/Library/Application Support"
PlexTempDir="/tmp"
LangEncoding="en_US.UTF-8"
ExistingVersion=11901
HaveHardware=1
NeedUser=0
NeedGroup=0
NeedVideo=0
Verbose=1
Running=1
cat /etc/default/plexmediaserver # default script for Plex Media Server
# the number of plugins that can run at the same time
export PLEX_MEDIA_SERVER_MAX_PLUGIN_PROCS=6
# ulimit -s $PLEX_MEDIA_SERVER_MAX_STACK_SIZE
export PLEX_MEDIA_SERVER_MAX_STACK_SIZE=3000
# where the mediaserver should store the transcodes
export PLEX_MEDIA_SERVER_TMPDIR=/tmp
# uncomment to set it to something else
#export PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR="/var/lib/plexmediaserver/Library/Application Support"
# the user that PMS should run as, defaults to 'plex'
# note that if you change this you might need to move
# the Application Support directory to not lose your
# media library (match what is in /etc/passwd)
export PLEX_MEDIA_SERVER_USER=plex
# Uncomment this to use syslog for logging instead of
# sending logs to Plex Media Server.log
#export PLEX_MEDIA_SERVER_USE_SYSLOG=true
