Thanks for the reply I-See-Light!
Looked at the file rights and Lightroom had access to Downloads, Documents and Desktop. And so, I was off fishing again; this problem has been showing up intermittently for a while and I resolved it by putting the raw files to import into a separate directory for import---but that was too messy a long term solution.
After some time at the screen, what proved to be my issue was the presence of .MOV files in the directory that I was trying to import from. The directory has 3284 .jpg / .cr2 files and originally had 22 .mov files. And so, when trying to import the whole directory I sometimes returned a dozen or so previews with the rest blank and dark gray---other times it would return all blank, dark gray previews. Removed the .mov files, and all worked fine. Does look like Catalina and Lightroom are not happy together.
Thinking I may need to play around with multiple boot partitions, although that will require some time to figure out. And was thinking about upgrading to Lightroom 6.14 if I could find it; but don't think I want to risk that at all...6.0 has been working fine for me for a few years now.
Thanks again,
David