While this may seem argumentative (and I don't mean it to be), but can you confirm this
Your post on June 5th says the same thing
but the only diagnostics that you've posted recently that I found was from June 9th (https://forums.unraid.net/topic/80680-checking-for-docker-app-updates-spins-forever/?tab=comments#comment-750259) which shows that appdata is stored on cache, disk1, 2, and 3. While this is perfectly valid if you only deleted the plex folder (you did delete it didn't you?), it does *imply* that the possibility exists that the entire plex folder doesn't exist on disk3.
And, according to the diagnostics on June 9th, your BIOS version is still from 2015, but numerous updates have been made to it, with the most recent being June 25, 2019. Not saying that this is the issue, but an update would at least hopefully rule that out.