I read in the past that there is a problem in Avidemux (approx v2.4.5) when appending two *.mp4 files.
The audio track was not correctly continued resp. cut.
Is this problem solved meanwhile?
No idea about anything pre-2.6, but the requirement that the number of audio tracks, codecs including codec parameters used and channel layouts must exactly match persists.
Especially if the appended video starts not at zero, which is always the case when the video codec uses B-frames, a brief gap in audio is unavoidable.
There have been a fix for audio delay computation in the mp4 demuxer recently, you may want to test with the latest nightly.
@fhpro, please try the latest nightly. The error you see indicates that the particular video is corrupted. Current nightlies should not crash when they encounter such a condition, they just refuse to decode the video at this point.