Cannot open MP4 file

[mlum]

I converted my mkv video file to mp4 file and when I open it in Avidemux, I get a "Could not open the file" error.
How do I fix this?

[eumagga0x2a]

First of all, please verify that the issue persists with the latest nightly build from here or from here (as of this writing, it is r211112, i.e. from Nov 12, 2021). If it does, please post textual information about the source mkv video and the generated mp4 file provided by MediaInfo.