Thanks. But it didn't work.
Is there anyone with an other solution?
Try deleting the preview cache, it could be corrupted.
Close Lightroom, then use the Finder and go to the folder that contains your Lightroom catalog - usually inside the Pictures folder.
Delete the folder whose name ends with Previews.lrdata (it is possible that this folder appears a file on a Mac, but it's the same thing)
Do not delete anything else.
Now launch Lightroom, and the previews will be rebuilt as you browse the Library.
You can also tell LR to rebuild all previews (Library > Previews).