Hi Harald Braunstein,
The problem is in the Silverlight framework. It appears that if the popup is not in the visual tree (the case with RadContextMenu) and Vertical or Horizontal offset is changed and there is a Zoom then this popup instance is not working as expected - it is not zoomed and its size is wrong. This is why the placement of RadContextMenu is wrong. We have contacted Microsoft about this issue and we will let you know once the problem is resolved.
I'm sorry for the inconvenience caused.
the Telerik team