Lukas,
Interesting. I ran the same very simple null->null and it did open another command prompt window and, well, did nothing, which was expected.
I do have the following extra line displaying:
> gr::pagesize: no info; setting pagesize = 4096
and then after I close the window I get the same "Done" but no return code provided.
So obviously something is failing early in the python script before it really gets the flowgraph going. That error code is fairly vague, google tells me it's a generic system error.
Please try this, open the GR command prompt, and attempt to run the generated python file manually, and see if it produces any more useful detail. The next step after that would be to try to run the python script in Visual Studio (or your preferred debugger) in debug mode and figure out what line is causing the failure.
Another thing to try to would be to run it as admin and see if anything different occurs. Also try setting to "No Gui" just for kicks.
The fact that the second flowgraph does show the radio initializing is a good sign, it shows that block is starting up as expected.
Geof