I know this topic is a little bit stale by now, Bit in the meantime I have discovered what issues the original poster might have had. So I just writhe my thoughts here for reference:
Some print shops offer their soft proofing ICC profiles only as CMYK profiles and then the darktable has an issue with displaying the soft proof (I stumbled upon this issue recently). As a quick workaround it helps to use gimp for soft proofing in this case.
e.g. the ICC profiles here: cyberlab.at | Das Hi-Tech Fotolabor exhibit this behavior.