Hi Michael,
unfortunatley I can not send any files, since i am here under a heavy "Non disclosure spell" ...:-(
But i will try to recreate the problem with another model which i can use more freely.

Your tip with the OBJ converter was worth a million, because it seems as if it not only solved my
vanishing objects problem but also my importing issue. Thank you a thousand times!

One more question: Is it possible to convert curves from a curve with edges into one smooth one
with direct or indirect control points?

Best regards,
Heiner