Thanks for your answer.

Unfortunately, I already launch Mumble before launching BC2, and I can't set the game to DirectX 9 (or I don't know how to).

By the way, it seems that some of my teammates are also experiencing this overlay problem with BC2. I asked them to give me more informations about their computer, and I'll post those infos here asap, it might be of some help to find where the bug comes from.

Edit: I just thought about it. I tested the overlay with Dirt 2, which use DirectX 11 too, and it worked fine. Only BC2 doesn't work.