Looks like I had messed up some settings:
Printing something with two colors (UM3) was no problem - until the day before yesterday.
Imagine: Head 1 white filament, head 2 blue filament. Printing a white square, 5 mm thick, a blue triangel in its center, 2 mm thick. The layout (in Tinkercad) is perfect. Las week it has been printed perfectly -- front a white square with an embedded blue triangel, back all white.
Now (and I don't know why) the first layer is being printed completely by head 1 -- although it's printing the triangel seperately as if it would have liked to