Hi there,
Well your problem has absolutely nothing to do with the cartridge and preamp matching!
The fact that you actually had some success at first shows that this part works fine and yes the MMC 20CL is an (excellent) high level cartridge!
Do you see the red stand-by dot?
Now there might be some problem with your DIN to RCA adapter shorting the Datalink pins of the plug (the two extra pins inside the plug).
If you don't see the stand-by led there might be a bad connection inside the transformer of the Beogram. This is fairly easy to check because it will just slide out (no screws). Open it and check the white connector.
Otherwise, you will have to go deeper into the deck to look for bad solder joints at the transformer connector which is very common in these decks!