I had the exact same problem, but after reading your comments I realised that it all had to do with the external display's Colour Profile. Illustrator and InDesign just refused to work with the custom profile I'm using.
When I reverted to the monitor's default Colour Profile, everything worked like a charm again.
I had the exact same problem, but after reading your comments I realised that it all had to do with the external display's Colour Profile. Illustrator and InDesign just refused to work with the custom profile I'm using.
When I reverted to the monitor's default Colour Profile, everything worked like a charm again.