I just upgraded to Lightroom 6 (from 4) and when trying to import instead of previews it says "Preview unavailable for this file" on each photo. If I then move to another window (like this browser for example) then the text disappears but they are just gray, no previews (really looks like a bug). Never had this issue with Lightroom 4. I have to see the previews because I don't always import all the photos from a card, I choose specific ones (as I've always done with Lightroom).
Do you have installed the latest version of Lightroom (it's LR 6.4)? Have you tried to disable the GPU support?
Got to the menu Edit > Preferences and then to tab Performance. Then disable GPU acceleration.
Hope it helps.