SOLVED: Playback Error

[TheDreadPirate]

Remuxing is taking the contents of a container (MKV) and copying them, unchanged, into another. A container is not just a file extension. The MKV/MP4/TS/AVI, etc., containers provide metadata and time stamps for syncing up audio and video, provide mechanisms for multiple audio/subtitle tracks, etc. If the container is malformed that can cause playback issues, even if the actual audio and video are fine.

The error in your ffmpeg log appears to indicate a MKV container problem. The ffmpeg command I provided should copy the contents of an existing file and insert it into a new, properly generated, MKV container.