After repeatedly getting Muxer error, I stumbled upon this combination of settings for the latest (2025 Jan 1) version of Avidemux on a computer with Windows 11 OS:
Video Decoder Lavcodec DXVA1
Video Output: Copy
Audio Output: Copy
Output Format: AVI Muxer
Outputs .mkv file
Have no idea if this helps anyone. But, when you find something that works, tell the world!