After looking inside of the machine, I noticed a motor, with just a " bare " shaft sticking up.
I installed the above mentioned disc on the shaft, with the cd-centering protrusion facing up.
This was quite simply achieved with needlenose pliers.
I figure this out after looking at the disc & comparing the centre protrusion.
Sure enough, same size as the centre hole of a cd.
It stays in place on the shaft quite nicely.
Still the same error message.

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2007 C 230 Sport.