Web browsers and app require media to be transcoded in a format like AVC (H.264). Formats like HEVC does not play within browsers, as far as I know. You need to. turn on transcoding if you want to make files load 100% of the time.
I set up transcoding according to the guide and enabled hardware acceleration, per user recommendation
This should not be happening, then. How have you set up Jellyfin? Is it in a docker container? If yes, how have you passed through your GPU? Have you tried turning off Hardware Acceleration? In which format is your media stored?
No, it's a Windows install on the server and the client. Hardware-wise, the server is overkill. My libraries are in several codecs and formats, and everything seemed to work indiscriminately until recently but I didn't make any admin changes.
Shorter version: playing a video in your browser or in an app that uses webview is different than playing it in your phone media player.
Doesn't the Jellyfin app let you play in an external app? If so just make it play in VLC, it plays anything.
Check your ffmpeg logs that should help find the cause
Restart Jellyfin and verify that the media is accessible
Jellyfin: The Free Software Media System
Current stable release: 10.10.0
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