

Do you think you could post a picture of the exact error message? Also you could check the Plex logs through the web app and you should see the exact error and what component caused it. If I had to take a guess, I’d say your uncompressed movie has something that’s causing the server to transcode (TrueHD audio or a forced HDMV subtitle stream) and the transcoding process is failing.
I don’t know how it impacts the paid feature changes recently but you can add additional “LAN Networks” to Plex in the “Network” tab in the Settings to consider additional IP Ranges as a LAN network. I believe the LAN detection is based on the original subnet of the server where traffic from any other range is treated as WAN access.