Hello!
I had the same problems when I tried to use an imported scene, created in a CAD software, even though it had an extenssion like the ones recommended in the User Guide. I don´t know the answer to your questions, but I can tell you what did I do in order to be able to work with an imported file in I-Simpa.
You can see in
this post a recommendation I received: I was told to use an specific software to create the scene and everything worked well when I used it in the simulation. I can add the fact that you can find the option to save it as an STL file, in order to use it in I-Simpa, by clicking the "Export" option in the Form It software.
I hope it may help you. Bye!