The only remaining item, that was buried in the middle, is: Is it a single NIC on your server?
The reason I ask that is it seems from my observations that anything beyond a single NIC where Plex is concerned can result in any manner of odd issues relating to connectivity and reliability. If it is not single NIC, then I would attempt that change to see if it helps. If it is single NIC, proceed below.
(Actually one other random thought: do you consider your router and modem powerful enough that they aren’t the bottleneck / potentially choking with the outbound throughput?)
And yes, the next step at this point seems to be to try and get some direct support. Often that can be a little luck but in this instance I would suggest attempting to summon ChuckPa into the thread. They are very knowledgeable especially on the *nix side of things. I don’t want to @ them until you’re ready though.
So, my suggestion would be to follow the steps outlined here https://forums.plex.tv/t/help-us-help-you-how-to-provide-logs-to-get-your-problems-identified-and-hopefully-resolved/672904 to generate a full log set, upload them here and then @ ChuckPa (alternatively if you don’t want to share your logs publicly, @ ChuckPa first and request to provide them privately). Hopefully they may have some existing knowledge with the errors you are encountering, or be able to discern something pertinent from your logs.
