Thumbnails not showing | WordPress.org

Hi,

I have several galleries and since some recent updates, the thumbnails of each image do not appear. I can see there’s a blank space and when I click on it, the big image appears with the left/right arrows.

I use WP-Rocket with Lazyload unable and SMUSH (from WPMU) with lazyload disable.
I’ve emptied the WP-Rocket cache.

So, have you got an idea for this non appearing thumbnails please ?
Some pages show them but it’s very very slow

Thanks

Patrick