In the container path /config/log there will be a bunch of files titled FFMPEG-Transcode<random string>.log. Pick one and attach it to a post or post it to sourcebin.
Jellyfin 10.10.3 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)