I'm having the same issue and I don't have Hummingbird downloaded. I have WP Smush and I tried deactivating it, but the problem still occurs. I can't edit anything in Pro or see anything in the preview.
JS and CSS minification are deactivated on WP Rocket. The editor started somewhat working but when I tried to load it fully, I get this message:
The preview was unresponsive after loading. This is most often related to a plugin conflict or aggressive page cacheing.