I am sorry for the inconvenience caused to you.
Please follow the below steps to remove and create preview cache :
> Quit Lightroom
> Navigate to the location of the catalog
> Rename the folder Previews.lrdata to Previews.lrdata.old
> Double click on Lightroom catalog (.LRCAT) file
Let me know if this helps.
I tried that, but it didn't work. Still no thumbnails showing in the
Import dialog, just grey boxes with filenames and checkboxes.
I tried that yesterday (before posting to the forum) and it didn't help.
This is common it appears. After you've done the stand Mac troubleshooting: repair permissions, restart. Simply do this:
Open Lightroom and the catalog in question. Go Under the FILE dropdown menu and chose "Optimize Catalog".
This should fix the problem.
All the Best,