Please follow the below steps.
Go to Lightroom Preferences from the Edit menu
Click on Performance tab
Uncheck the Graphics Processor
then, Go to General tab
Uncheck the option " Show Add photo screen"
Let us know if that helps.
Thank you Mohit for the fast reply.
Having followed the recent LR update user response, I had both of those unchecked as soon as the update was complete. Those check boxes should not have been the source of the anomaly.
What is odd, is that something changed during the last two hours of sending RAW files back and forth between LR and PS for the course I'm working on. When I wrote the inquiry here, the edit dialog box had Photoshop and Photoshop Elements in the top two lines. Photoshop Elements has been replaced by a third party plugin and is nowhere on the list of choices.
Better, it decided to work! JPEGs open using the "What to Edit" box with the usual three choices. The plugins work too!
I'm not new to LR as I started learning it with 4x, worked through 5x and joined in on the CC for 6x to get a chance to learn Photoshop. I'm embarrassed to have asked for help on something that fixed itself so quickly!
Thanks again for your help.