I have a Pro account with mmm very large number of images. And this had become a problem. Backend simply would not load the images unless i cleared the cache and reloaded the page multiple times. I was in fact considering switching to a brand new account to resolve this problem at my end.
But in the last 2-3 days I have seen that this problem has largely disappeared. Images load fine after a slight delay. Funny enough, this behavior started when I renewed my Pro account.
Just wanted to see if you had changed something in the code that was used to fetch the existing images for display in the backend!