I've put the ICC-profiles into the $/home/USER/.color/icc/ directory (I had to create it) as kindly suggested by the tool-tip of the ICC-profiles-field in Scribus: Preferences --> General Settings. Maybe I can do the same thing on Windows.
After restarting Inkscape, the color profile dropdowns now work. A small tool-tip describing the preferred folder path on either of the dropdowns (like in Scribus) would have shortened my research significantly, to put it mildly...