Well, I think there really are bugs. I can put the layer of the canvas wherever I want - in front of the others or Behind them - make the layer invisible or visible, make the canvas invisible or visible, set it to draw before of after drawing, and even make it draw a dummy line just to say it did draw...
Nothing works. It always result with a white picture saved by the imagemanipulator.
There really is no other way to screenshot, huh?