I cannot recall when or why I should have applied a Softproof adjustment to images in my ID CS6 document, but suddenly they appear, when I open the IDML in APu. I made some other tests and with newly created documents I have no problem. I could upload the package to a private dropbox if wanted.
EDIT: I think I got it. I always get the Softproof adjustment, when I place an unprofiled image in ID and save as IDML. Is this a wanted behaviour?