Looks like the QucsStudio forum censors the posts, so no luck there.
Looking at the forum homepage the last post in the "Usage" section was by user JOEQSMITH 2 weeks and 2 days ago, which cannot be correct, and your posting is not displayed when I open that section. To me this looks like a technical error. Have you tried contacting Michael Margraf?
Attempting to manually create the PCB layout, many of the library components are not supported, which includes coplanar. I created a new schematic using microstrips which are supported. If I add a capacitor (you have to use the lumped model) and wire it across a microstrip gap, the software will not include it when creating the layout. You can however add the capacitor to the layout directly.
Including a capacitor works for me, with and without including a microstrip gap, which I believe is not necessary for the EM-simulation. As for coplanar waveguides, I think they are currently not supported in the EM solvers layout creation tool. But you can run a regular S-parameter simulation with the built-in coplanar waveguide models, together with lumped components. This may or may not be a good approximation to a real EM-simulation. For these conventional S-parameter simulations you can also include S-parameter files.
The simulation starts without any errors and made it to 87% complete where it has been for about an hour now.
I'm by far no expert when it comes to EM-simulations, but you can try to modify the simulation parameters when the simulator hangs (under File -> Document Settings). In the "File Properties" dialog box there is a tab "Excitation". Set "Energy Decay at End" to a larger value, as this determines the termination condition of the simulation. You can also try to modify the mesh size under "Domain". And you can always hit the "Terninate iteration" button and get the result from the current iteration.
Even if it would run, the software is lacking the ability to use S-parameters capacitors. The next problem I see is that I am unable to find a way to modify the ground plane.
As I already said, I believe S-parameter files are only usable with regular S-parameter simulations. I have no idea how to modify the ground plane or if that is even possible from within the layout editor. Maybe only if the layout data is created externally and then imported? The underlying solver is 3D, if I'm not mistaken, so it should be able to do that. I have never used qucsstudio for such complex simulations though, only for simple microstrip filters.
Or one could try to use openEMS directly:
https://openems.de/start/. Again, no expert here, and I have never done that, and there is nu GUI.
I really would like to take a deeper look into the capabilities of qucsstudio, but I have virtually no time during the coming days.