I've been a long time user of Avidemux though I'd hardly call myself advanced. It has just worked and worked very well.
I have used it on a Win10 pro computer without issue, however rather abruptly I receive a message that the muxer cannot open when trying to write an mp4 file to mkv. It doesn't seem to matter what .mp4 file I load...I always get this error message using ver 2.7.5 I uninstalled 2.7.5 and installed 2.8.1 and have exactly the same problem.
I'm unsure how to progress this sudden change in behavior. Thanks in advance for assistance.
First of all, v2.7.5 is ancient and absolutely unsupported (v2.8.1 is also ancient, to be honest). Please remove v2.8.1, delete its program directory, then re-install (rather the latest VC++ 2.8.2 nightly from https://avidemux.org/nightly/vsWin64/ than the latest release from soon 3 years ago) to an empty directory.
If the problem persists, please start Avidemux afresh, reproduce it, close Avidemux and provide admlog.txt from %localappdata%\avidemux\ directory (you may sanitize it by consistently replacing paths with placeholders first) as an attachment to your reply.
Please also make sure you never run multiple instances of Avidemux concurrently.
Thank you very much! Problem is solved!