westfw:
In a Gerber viewer, the layers seem to be empty,
Ah. ALWAYS check your gerbers before sending off a design...
Yeah ... 
Luckily, it was not important for either of the projects. Still, for the next one I really would like to know what I did wrong.
@pwillard
That's another possibility. Looks fine. But again, it is only 10$ for 10 boards, and I do not have SMD headers at hand.