Newbie so be gentle and redirect if appropriate please ;).
I've been using Avidemux to concatenate .avi files from a Garmin dashcam. 1 minute per clip so can be quite a few clips to concatenate. Mostly it works fine. Not infrequently though the output file can't be opened by anything (eg VLC) or even by Avidemux itself even though the concatenated video seemed just fine prior to saving. Seems quite random - the next time it will maybe save just fine. I'm using settings out of the box just selecting .avi for output. Windows 10 64-bit current version of Avidemux 2.6. Is this a known issue?
No, because no one can guess what the actual issue is. If the issue persists with the latest nightly (http://avidemux.org/nightly), please try with MKV as target container. If it fails too, please provide source files as samples.