But this solution is valid picture by picture.
I have a better solution :
with development mode, i select one picture after import and I reset it.
with Library mode I sync parameters of exposition over all pictures imported.... and the pictures appear.
But why after a month this problem is not resolved by Adobe ?
I am sorry for the inconvenience caused.
Here is an article for this : https://helpx.adobe.com/lightroom/kb/thumbnails-display-black-in-lightroom-6-2.html
This issue might be fixed with next update.