Images Issues with a WooCommerce and thumbnails regeneration

Hello all !

For any reason, something about woocommerce product images broke in the store that we're building now.

Suddenly we saw that adding a new product, the image was loading fine in frontend and after 1 second new product's image disappeared.

Now after regenerating manually images, system show them correctly, but after adding a new product, most of images disappear now, very funny, see this video:

We're using Porto and some customizations from an external coder.

We sent an email to Porto and they said this:

It's your server issue not the theme related issue. I have tested the theme but the theme is working fine with product images. This issue is only on your site. Your server / site did not generate the thumbnails of your images correctly.

And sent us this image: https://snag.gy/xbr2Hc.jpg

We have spent all day with this issue and no one is abble to tell us what's going on.

Any expert can give us a hand please ? Thanks a lot.