I did name the error right in the first line of my post: "File Loading Error" is all it gives me.

I reinstalled the software, since I couldn't open any of the sample models. I can now open them without textures, but have the same problem as with the selfmade stuff: It won't allow me to preview it at all. The window in the preview is just blue and nothing else is in it, even though building the map itself won't give an error.

I will try to open the models the way you suggested and without the material settings later today, and let you know if it worked then.

Thanks so much for helping me:)