croppadd doesn't work with convert/save media
As soon as you use croppadd to remove unneeded margins of the movies, the converted video includes only audio.
I think I have identified the problem in the log:
croppadd debug: Crop: Top: 75, Bottom: 75, Left: 11, Right: 11
croppadd debug: Padd: Top: 0, Bottom: 0, Left: 0, Right: 0
croppadd debug: 720x576 -> -22x-150
It seems to be an initialization problem.
First I thought it affected only the convertion from a capture device ... so I opened an other ticket: https://trac.videolan.org/vlc/ticket/10148