Hi,
I am guessing, that WP Rocket is moving the image URLs to a cdn subdomain, and it is probably conflicting with the cropping in some way. Maybe the server does not have privileges to parse contents from subdomain URLs or something like that.
Anyways, I think the best option here is to simply use a custom code to try to replace the images with the thumbnail versions. Add this custom code to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case!
This should do the trick.