Yay, finally, I am able to create properly formed upload files which means that the app can now at least upload checked tasks and OmniFocus for Mac properly recognizes them. It took far more time than I had initially expected, but finally this part is done. This does not mean that its completely done but at least the basics are in place.
The next steps is going to implement the basic UI for the application, and eventually I need to make it pretty. So there is still some time until I have something to show in public but this was a very important and complex step and therefore an important milestone.