I found some solutions for this kind of error in the internet, like renaming the preview Folder or deleting it. I tried both, but it does not change anything, the same error occures anytime I try to start LR again.
Thanks a lot.
Please update Lightroom 5 to the latest version 5.7.1 if haven't yet.
We have documented a solution for this issue here Lightroom error on preview cache or displays gray thumbnails but what you need to make sure is that you're deleting the correct Preview cache, it should have the same name as your catalog.See in the screenshot below, the preview cache have the catalog name followed by an extension.So,before deleting or renaming the preview cache, check it belongs to the current Lightroom catalog in use.