I think this is a side effect of /tmp filling up.
I’m running on Debian so I used this from what I gleaned from the discussions around this issue:
Add following to override.conf in /etc/systemd/system/plexmediaserver.service.d:
[Service]
Environment="TMPDIR=/srv/dev-disk-by-label-data1/plex_media_server_tmpdir"
Oh, and just to be ultra ultra clear, the dir after TMPDIR= is wherever you want your Plex temp dir to be. This is just an example.
Since I made this change all the errors I reported have gone away. So far.
I hope @ChuckPa will chime in one way or the other.