Strange. I`m quite sure my old AV9000, the early two way version, and MPAV were connected by a regular ML-cable......(?) If I remember correctly, there are two Masterlink type plugs. First for connection of TV with control center, 2nd for MPAV and for link systems. Are you sure, everything is properly connected?
If this doesn`t help, there is an AV9000 expert in Germany, Ulf Kiener, a Beoworld member mainly in the german forum. Member name is "Ulf".
See also: www.ulf-kiener.de. If he can`t help, nobody can....
Good Luck
Stefan