Mac OS X 10.7 App Store: "Unable to complete your purchase"

When I try to install an already purchased app, or a new app, I get the error message: "Unable to complete your purchase. Unknown Error".

It doesn't matter which app, paid or unpaied. I've already reset my warnings through iTunes, signed out and in again, restarted, etc.

Is there a way to reinstall or "clean" the "App Store" App?

Roeland