Simply merge several videos into one

I tried all kind of settings but I fail and get weird results.

I simply want to merge multiple video-files into 1 videofile.

What I am doing:

  1. I add the first file into the app (part 1/13)
  2. I do rightclick on the file and select (append files)
  3. I choose the other 12 files to apped these
  4. I start the process by pressing the button “Start multiplexing”

The result is not what I want. The video is full with problems (no content showing after a while, only sound)

What am I doing wrong here?