This appears to be caused by a small issue in the code, which specifies network path to the subtitle file incorrectly and the workaround solution is here.
When you start the file stream Plex attempts to retrieve the file location and subtitle, like:
- Request film [file://\\Server\Media\abc.mp4]
- Request sub [file:// Server\Media\abc.srt]
The missing “\\” in front of server network path for srt file, causes an issue.
Solution is to re-map the source of library from “\\Server\Media” type of reference to “Z:\Media” and run a full metadata update.