Headset issue

Hello out there!

Today my headset buttons suddenly stopped to work, and i don't know why.
I read that some apps could be the problem, so I deleted every single app that I had installed on my phone. That didn't fix the problem. Then i tried a hard reset but no luck with that either.

Im running Android 2.2.1 on my HTC Desire HD.
Music can be played with my headset, but nothing happens when i press the buttons. I cannot answer call etc.
(The microphone in the headset doesn't seem to work either...)

Can anyone help me out? What do I need to do, to make it work again .. I don't think my headset is the problem ..