I found out something new. At least in my case, it doesn't seem to be a faulty Bluetooth-device at all. I found someone with a R60, which has the same BT-module. I exchanged the modules and mine DID WORK in the R60, while installing its module in my Z60m made no difference whatsoever.
Do I have to conclude, that the wiring of the BT-module in my machine is faulty? Or could it still be a software issue?
I really would appreciate a Lenovo staff statement.