Philippe,
sorry, but there forums are almost 100% fed and handled by Dlink product end-users. There are only a few D-Link people responding to these questions, so yes, it might be disappointing, but if you read the welcome feed, you will also see that we try to help each other.
As most people do not have a 5220 the response will be few and far apart. So sorry, but that is life.
But I applaud you way of describing the problem, analysing it and finding the solution yourself. Probably you only needed a sounding-board, but this one was very very quiet. Alas, cannot be helped.