Interesting, and probably explains why remuxing the MP4 into MKV fixes it.
You setup is very similar to mine. UniFi Switches, QNAP TS-877 w/1060 6GB GPU and Shield TV for playback (all 1GbE) wired.
I donāt see any of the playback issues though that you are seeing.
Any suggestions on where to look from the log files I sent then, something isnāt right clearly and if itās not doing it for you then itās solvable for me somehow!
Edit:
Done something drastic, uninstalled PMS, deleted all leftover data and have reinstalled and now re-adding media.
There might be a couple UniFi controller settings to look at as well.
Iām out, but I played through one film this morning before I went out and it played all the way through, I also started ready player one both upstairs and downstairs before I went out and checking tautulli it looks like upstairs has finished playing and downstairs is still playing.
Donāt want to curse it, but it looks like itās behaving after a clean reinstall, I wonāt really know until I get home and check whatās on the TV!
Keeping fingers crossed.
Could a corrupt database explain this, I did check and it said it was fine before I deleted everything, was there any clues in the log files?
So far, watched a film tonight all the way through without issue, both ready player one sessions from this morning worked and Iāve started upstairs playing it and downstairs and so far so good.
Donāt want to tempt fate, but I couldnāt play a film through before without it crapping out with the server too slow message.
I didnāt see any DB errors in yours, but there are a lot of things tracked/updated in the DB/Library/etc. so you could have had fragmentation/churn impacting things.
I did see a lot of these errors though:
16302 Feb 12, 2019 21:44:55.404 [0x7f9a0bfff700] ERROR - SSDP: Error parsing device schema for http://172.29.13.132:49153/description1.xml
16320 Feb 12, 2019 21:45:05.443 [0x7f9a0bfff700] ERROR - SSDP: Error parsing device schema for http://172.29.13.133:49153/description1.xml
16324 Feb 12, 2019 21:45:06.471 [0x7f9a0bfff700] ERROR - SSDP: Error parsing device schema for http://172.29.13.134:49153/description0.xml
16349 Feb 12, 2019 21:45:25.233 [0x7f9a0bfff700] ERROR - SSDP: Error parsing device schema for http://172.29.13.132:49153/description2.xml
What do those IPās translate too?
Humn, Sky Q boxes (uk satellite tv system), quite why these are appearing in the Plex logs is a mystery.
You might want to look into this some⦠https://community.ubnt.com/t5/UniFi-Routing-Switching/Sky-Q-network-paths-and-STP/td-p/2117608.
There maybe other newer updated links about sky q / UniFi though.
I moved some of my IOT stuff into a separate VLAN to keep some of the multicast stuff out of my main vlan.
Also I would disable the 2.4Gh Hot Spot feature in the Sky Q boxes.
Both boxes still playing without issue, i definitrly would have seen a problem by now.
Only thing Iām seeing is the bug where the Plex server thinks the shield is on the wan and not lan so that you have to set the remote settings to original/maximum otherwise it will Transcode over the lan.
I keep meaning to move IOT stuff to a separate VLAN, but well, lazy.
The Sky Q boxes are connected via Ethernet with the WiFi disabled on them.
Hmmmmm⦠double check the PMS Settings > Network > Preferred NW interface.
Otherwise get logs from both server & client side 
Iāve set the Ethernet interface and also set the lan subnet. This bug has been reported, not just by me, whether or not it ever gets fixed is another matterā¦
Edit:
Hereās one thread about this issue from last year, itās a long standing bug.
Btw, Iām also on a 172.x.x.x range.
Are you within this range: 172.16.x.x-172.31.x.x??
I have pinged the gang on this to see if they knew if it had been addressed also.
Please get me debug logs from both your PMS and the ShieldTV showing that it is treating the ShieldTV as remote.
I can then use those to open an internal problem ticket on it.
172.29.13.X is my range.
Spoke to soon, went up stairs and ready player one was stuck. This is really annoying.
Iāll get you some logs tomorrow, Iām too tired tonight to do this and itās a pain on the phone, when Iām on my laptop iāll grab them for you.
sounds good.
Watching this thread and others closely. Not trying to hijack this thread but having similar issues. But it is more reproducible on when it occurs. Almost seems like it is related to some buffer or bitrate spike/drop. If you play the same movie, happens on basically anything with a high bitrate, each time it will buffer and/or get the message at the same point in time for that movie. Each movie will buffer a different point in time. Not sure if its related to this same issue or not. I can DM log messages if you like or can start up a new thread.
Client: Android 8.0.0; SHIELD Android TV Build/OPR6.170623.010)
Shield Experience 7.2.2
Server: 1.15.0.659
Connection: 1Gbps
Iperf3: 418 Mbits/sec to source file : Checked via computer connected to same ethernet as shield.
Bitrate of file per xml: 86109 kbps