Nicolas,
The problem you are describing is called "Vision on Sound" in the TV industry which does not happen very often in modern TV's but was very common in years gone by.
The cause is normally to be found in the IF stage where the combine sound and vision signal is split into its seperate components. You did not say, but I am assuming that this does NOT happen when using a STB. If it also happens with a STB, then the fault will be very different.
Unfortunately or fortunately depending how you look at it, all the IF stage these days are usually in one chip and therefore changing that chip and/or its associated components would almost certainly resolve the issue. HOWEVER many years ago B&O introduced field replacement units only, meaning component parts are are not changed by dealers only modules or chassis.
So, the short answer is the module or chassis will need to be changed by the B&O dealer if the problem is not using a STB. If you recently purchased this TV as a pre-owned unit without warranty, then it could be quite expensive.
Please confirm if this problem does NOT happen when using a STB and only happens with an internal DVB module ?