This is an update to the original request for user help, as I have isolated the cause.

If ColourSpace is closed with a Quick Profile that doesn't have Ramp Size capability selected as the active Mode - such as Memory Colours - the Ramp slider will not then be available for any Quick Profile, and will be greyed-out, although the values box will be active and can be used.

To recover Ramp slider operation, set the Mode to one with the Ramp Size option, and close and reopen the Profiling window.

Steve