Our engineering team is working toward a fix for this issue. Until then, use this workaround:
This issue occurs when Auto Tone has been applied to your images.
For existing photos, select all your images in Grid view, and then click the Reset All button in the Quick Develop panel. Then, click the Auto Tone button to properly apply this feature.
To prevent this issue from occurring in the future, choose Lightroom > Preferences > Presets (Mac OS) or Edit > Preferences > Presets (Windows). Deselect Apply auto tone adjustments.
Related Link: Thumbnails are black in Lightroom CC 2015.2/6.2
Let us know if that helps.
I have a variation of this issue...all of my imported pictures go to blue with a big X across the screen. Previews are viewable, but when I click on it in the Develop tab - goes blue with the X. I upgraded to 6.0 yesterday and have had nothing but problems. Very frustrating. Thanks for your help.
Sorry...I tried the above suggested fix - didn't work.
Thank you for that. I followed the steps as stated by you for my Mac and it worked. I'm sorry to hear it didn't work for 'cpossehl' though as I know how frustrating it can be. I've just returned from my overseas trip and am anxious to edit my images.