Well, with time comes wisdom.
The fact that the status returned a 0 should be the key for the awnser.
The awnser is that the command SYSTEM and linux go perfectly well together.

The problem in my setting was that the called script should produce a file.
I expected that file in the directory of the script, but it appeared in the bootstrap directory of the application (and I didn't notice it there).
Clearly a path problem which is adjusted in the script itself now.
And guess what?? It works!
So back at work.