Problems running 10.9.x releases

[TheDreadPirate]

When you ran the installer, did you run it as administrator? You are missing ffmpeg.exe in C:\Program Files\Jellyfin. Or Jellyfin isn't able to access it. Errors all over the place, mostly related to ffmpeg not accessible/present.

The issues with playback appear to be ffmpeg related as well. It is trying to do something with ffmpeg when you start a video. It may be trying to transcode, remux, or extract subtitles and fails. Because ffmpeg isn't present.

Try reinstalling and make sure to right click on the installer and "Run as administrator". Most users don't have to do that but, for whatever stupid Windows jank reason, some users do.