In the play list there were two short videos. One dragged to the timeline, made some adjustments and selected to export it, and it started.
In the timeline replaced the video with the second one, made some adjustments and selected to export it, it was still pending.
Than added a third clip to the playlist, cleared the timeline, dragged the third clip to the timeline, when started to move it on the timeline, the mouse pointer became a rotating small blue circle and in seconds program crashed.
Launched the program, in the timeline there was the second video clip, selected to export it. When finished got a black video, no picture no sound.
Launched the program again - everything OK.
What was wrong as I described in the beginning ?
Answers will be appreciated.