Import Error: Wings3D > Collada > Unity3D

I have a problem with certain models I created in Wings3D. I export them as Collada and import them into Unity3D. Some can be imported, but some can't. How can that be?

The Programm says: "ImportFBX Errors: Can read [C:Users/ ... .dae]" and "check external application preferences".

What does that mean?