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). Reselect Apply auto tone adjustments.
Refer to the related link: Thumbnails are black in Lightroom CC 2015.2/6.2
Let us know if that helps.
YES I have for last 2 weeks, installed anti crash update last night but still extremely slow importing or totally non responsive, and super slow to do any function. Have to force quit all the time. HELP