Please check the color profile assign to the external monitor, it could be the color profile has gone corrupt.
I deleted the color profile assigned to the monitor Acer K202HQL, and assigned Adobe RGB (1998). The problem remains.
You almost certainly don't want to stay with Adobe RGB assigned to your display -- it won't produce good results. See this article for how to properly troubleshoot incompatible display profiles: How do I change my monitor profile to check whether it’s corrupted?.
If after following those instructions, you still have the problem, try disabling LR's use of the graphics processor. Uncheck the option Preferences > Performance > Use Graphics Processor and restart LR.
Thank you for your assistance. Turning off the GPU solved the problem. It was working with the GPU on a couple of weeks ago, so either Apple or Adobe changed something. By the way, the link, in the Mac instructions, says to Select Expert Mode. That checkbox does not appear unless one holds down the option key when pressing the "Calibrate" button.