With editor like Avidemux you can cut a video only in correspondence with certain specific points (the Keyframes). Otherwise the video is corrupt. But there are applications that allow you to overcome this limit: for example ShotCut.
To use it a simple way is to add the video to the timeline (with “+”), and to set a cut point (with “][“), so splitting the video, and then to remove the undesired part (after selecting it). Then export the resulting video.