Playback broken: FFmpeg Error 254

[pr-o-xy]

Recently set up Jellyfin in a docker container on new home server, and getting this error whenever attempting playback of any media:

[20:11:36] [ERR] [111] MediaBrowser.MediaEncoding.Transcoding.TranscodeManager: FFmpeg exited with code 254
[20:11:36] [ERR] [111] Jellyfin.Api.Middleware.ExceptionMiddleware: Error processing request. URL GET /videos/2c7561fc-b33f-7fe6-6b1a-0856122890f7/hls1/main/-1.mp4.
MediaBrowser.Common.FfmpegException: FFmpeg exited with code 254

I've attempted enabling HEVC and AV1 encoding to no avail. Stuck on where to go next.

Jellyfin Ver: 10.10.6
Installation Method: Docker Compose
OS: Ubuntu Server 24.04
Hardware: Raspberry Pi 4B + 2TB mounted external hard disk