what to do?
If you are not going to use VapourSynth right now, remove libADM_dm_vapoursynth.dll from the plugins\demuxers\ subdirectory.
The popup error message is displayed on Windows 7 when Avidemux tries to load all demuxer plugins and VapourSynth, which the VapourSynth demuxer is linked to, is not installed. It went uncaught because on the test Windows 7 install, the current VapourSynth (R45), which includes vsscript.dll was present. The error is not shown on Windows 10.