Thumbnail Upload/Selection Error

Hi,

somehow thumbnails on one my website stop working :(. Everytime I try to pick/upload thumbnail, the blank space appears. I can upload other images, but it always select the blank tile.

Screen: https://www.dropbox.com/s/yspcp4g46k4b4dw/thumbnail%20error.jpg?dl=0

Gallery works fine, in media files there is no blank picture. It’s just when you are selecting thumbnail.

I try check FTP for some corrupted upload & PHPadmin for same, but I can’t find anything. No new active plugin, I tried deactivate all plugins and still not working. My WP is up to date.

Please help…

Pavel