OK. I cleaned all the files from LR and started from scratch. I followed
the instructions to calibrate the monitor. I imported a test file. Same
thing. Thumbnails are gray. Click one and enter Develop, photo shows.
Same deal. No luck. Still does not generate thumbnails.
Create a new catalog with a new name and see what happens. If your preview cache is corrupted you also might get gray thumbnails, and each catalog has its own preview cache.