Wow, that did the trick!
Is there any documentation on this where I could have found out this myself?
Anyway, many thanks, this helps a lot.
Important (path problem):
I noticed that if defining the file paths in the “Working Directory” field, then Grasshopper doesn’t know which folder the GH definition belongs to, and so it complains with an error message when trying to save the file with CTRL+S. (One has to navigate to the disk folder using “Save As” to resolve this).
Solution: But if using full path in the command line argument, then this probloem goes away.