Media Library png image not working

Hi, I’ve recently switched host and reinstalled wordpress. After reinstallation, importing back all the post data via database sql, and switching all images to a subdomain, I started to have the problem.
Media Library list view shows the image and the thumbnail, but the grid view (inside a post) doesn’t show thumbnail for png images. JPG and GIF are working fine.
This happens to newly uploaded png’s as well.
This means that I lost all featured images to my blog posts and all product images for my products. I cannot reset them manually, or create new posts/products, because when I select a png, it doesn’t show up.

I’ve tried:
disable all plugins, except woocommerce
replaced core wordpress files from a fresh download
checked htaccess file – it’s default
I don’t have any caching plugin and disabled cloudflare
Changed theme to twentyfifteen
Regenerate thumbnails

None of them worked.

Could someone please help?
Thanks