I have edited an MP4 video to remove frames and have saved it but although Avidemux reports the file successfully saved, the file itself is reported invalid both by Avidemux and by Quicktime when I try to open it.
As I can see on your screenshot "Avidemux 1.gif ", you didn't change the container format from avi (default) to mp4 (what you had and what you wanted to get). That may be the cause.
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Screenshot 1
Screenshot 2
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File Added: Avidemux 2.gif
Screenshot 3
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Originator: YES
File Added: Avidemux 3.gif
As I can see on your screenshot "Avidemux 1.gif ", you didn't change the container format from avi (default) to mp4 (what you had and what you wanted to get). That may be the cause.
I was experiencing something similar with Avidemux v2.6.5 on both 64bit and 32bit version.
Solution here (copy whole link): http://www.avidemux.org/smf/index.php/topic,12719.0.html
Last edit: KeMa 2013-09-08
There is an updated muxer dll available in the forum
http://www.avidemux.org/smf/index.php/topic,12719.msg65262.html#msg65262