thanks for your speedy reaction.
It is a new drawing in QCAD and I copy the circle into the same current drawing in order to duplicate an object.
I save it as *.dxf and my OS is W7 64 bit.
Now that you asked, it appears to me that the trouble starts after saving because I controlled the the distances after drawing. At that time they were o.k.
When I open a saved drawing, the measuring procedure will be as described in my mail, but funnily enough not in all cases.
Can't imagine it's QCAD, it must be me.
Regards and thanks