Could you please post screenshots to illustrate the issue?
What is the original ai file’s Color Space?
What is the Color Space of the file converted in Photoshop (or what are your Color Settings)?
Actually I can't post that original file in this forum it's an internal file. The original ai file is created with Adobe Ilustrator CC, CMYK as I already mention in my quetion. What exactly do you mean by Color Settings? I saved the pdf with the standart settings. I've got the feeling that Adobe Photoshop CC got a Issue by saving pdf as jpg because I never had that problem with Adobe Photoshop CS5. I also Importet the ai file to see if that makes any diffrences - but no. By printing the pdf and the jpg (saved from Photoshop) there are minor diffrences - jpg color are slightly brighter. Is that because jpg compares the file in points and so there is less information? No idea... I exported the ai file directly from Adobe Illustrator to a jpg but then the file is heavyer as in Photoshop made - of course. But then it works! So what the hell could be the issue?
CMYK is a Color Mode, not a Color Space.
What Color Spaces are the original and the converted file in?
Does the ai file contain RGB data?
What were the settings in PDF creation?