Nicolas,
Based on your "Problem Statement" it sounds like you have some king of interference or pickup from another source.
You appear to be saying that everything has worked without problems until yesterday, if so, then my first question would be:-
- Has anything changed in the surrounding environment in the room?
- Have any wires or connections been moved ?
Given, that the problem apparently went away when you removed the Beolab 3500 tends to lead me to think that you are receiving RF (Radio Frequency) interference which can often be more apparent in hot weather. Like most modern products, B&O products do not have a mains earth connection, so this does mean that large metal surfaces such as the case of the Beolab 3500 can make very good aerials.
The way to prove if your problem is RF interference is with the Beolab 3500 conencted and the problem on the screen connect an earth wire to the case of the Beolab 3500, if the problem goes away you know the problem is caused by RF interference.