APS question

In my V280 from 1998 there is an APS system. Nowhere to find what model it is but after searching the net I found it: it is APS 4.
Unfortunately I get "CD read error. Please change CD". Since it doesn't
matter what CD I insert, the error comes back. I understood that inserting music cd supposed to give another error: "No NAV CD. Please change CD."
Does it say that my NAV unit is broken??
Anybody knows what brand actually such an APS is? Blaupunkt? Becker?
I have to admit that so far didn't have the opportunity to test with original MB CD.
Greetings
Art
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