This article gives steps how to enable and collect Plex C# server log. The example is for Plex 7.3 and previous versions are similar.
Plex 7.2 and higher
Follow these steps
If the LogType is FlatText, logs will be written into flat text files (C:\ProgramData\CA\Plex\7.3\ob.Net\log); if it is EventViewer, then logs will be written into Event Viewer application log and log would need to be saved as evtx file.