I created a model in Sketch-Up (Sketch-Up detects it as a solid, a closed hull, where all faces can be seen from outside).
Then i exported it as Collada .dae.
But Blender splits 5 faces off and converts them into separate objects, each one out of two faces, one face lies flat on the other one.
This is the link to the original-sketchup-object
This is the model in Blender. Main part moved to set the separated objects :
Why does this happen?