When you have a chance, please stop/restart Plex Media Server, wait 3 minutes (for Plex to fully start), then pull a new set of log files.
The log files you provided did not have DEBUG level logging enabled at the last restart. There are several network timeout messages in the logs, but without the debug level logs, it is hard to tell what is happening.
You updated from PMS 1.18.5.2309, released Jan 2020, to 1.21.2.3943, released Jan 2021, 18 months ago.
Is there some reason you did not update to 1.27.1, the current release?
Note that when you do jump several versions it may take quite awhile for Plex to fully start. There will be several database changes and the migration can take some time.
