Windows 7; Avidemux 2.7.5 - first time install.
I've not used this before, used to use virtualdub.
I have a file encoded with MJPG, 640*480, 1:1 aspect ratio, 20fps.
Audio: 8bit PCM Mono, 11024Bps / 88kbps
The video plays fine in Avid, but when I try and play it, even just with Copy/Copy for video/audio, I get an error: Muxer - Cannot Open. Pressing Ok brings up another error dialog - "Failed - X.mkv was NOT saved correctly".
Thoughts? The search only finds one other result for MJPG.
The problem is most likely the audio track. Please test whether it works when you encode it e. g. to mp3.