Preview Image 404 error

Hi!

I have a problem with the preview image.
I download an image from admin page, the image is in the correct ftp folder with the good name thumbs_videoname.png, but it is not visible.
When I copy the URL link of the image, I have an 404 error.
Why?

Thks!

Cool Video Gallery v.1.9

https://ww.wp.xz.cn/plugins/cool-video-gallery/