My problem is sorted, but I was never informed by the dealer what was really wrong. After 5 visits and some calls to the chief director of AR, the problem was solved, but the reply of the dealers chief engineer was "I cant tell you!"
Either they planed the manifold's surface (at where the gasket fits) or they fitted a used manifold, taken by their own show room cars. Not even to think about warranty replacement. Which verifies the unofficial order to cut warranty expenses. Another member PMed me about the same problem, it seems its getting a common one..
@Patrick: Did you fix yours?