I also have this exact issue on my Mac Mini (late 2012) running macOS High Sierra 10.13.6
It is some old hardware and the OS cannot be updated any further, perhaps Plex has reached it’s limit for my hardware?
I also have a backup server on a windows machine that did not encounter this problem
edit: I have reverted to version 1.29.0.6244 which has got me up and running again.
I used the rollback instructions in this post:
https://forums.plex.tv/t/how-do-you-roll-back-plex-server-on-a-mac/737969
