Impossible to adjust the colour profile under OSX High Sierra

Since I have upgraded to MacOS High Sierra (OSX 10.13.6), the displayed colours of my iMac 27"are over-saturated.

I have tried to adjust them under Preferences -> Displays -> Color: although a lot of colour profiles are listed, only one is available for my display: sRGB IEC61966-2.1 when I tick "Show profiles for this display only".

When I use the Calibration tool (either in the Standard or Expert mode), any change I do in the settings is not directly visible on the display and the Display Calibrator Assistant ends up with the following error message: "An error occurred. The new calibrated display profile could not be created or set to the current profile for this display".

By the way even if the above error message has been displayed, the newly created profile is still listed in the colour profiles list but when I select it, the button "Delete Profile" remains greyed-out. I have been able to find it back under the folder Macintosh HD / Library / Colorsync / Profiles / Displays where the following colour profile is also available: iMac-A6AF052A-8EDD-649B-1034-6C7C9741DD45.icc.

I have then used the "Colorsync Utility" application and clicked on "Verify" under "Profile First Aid": a lot of the profiles were apparently corrupted. I clicked on "Repair" and since then none of the 152 profiles found has a problem, according to the application.

Moreover, any change of the "Color Temperature" under Preferences -> Displays -> Night Shift has absolutely no impact on the display of my iMac.

It seems that something (application and/or OSX) prevents from changing any settings regarding the display profile and that only one colour profile is applicable to my 5K Retina display. How can I adjust the saturation of the display colours ? Any help would be very much appreciated.