Thanks for sharing. I will definitely try this if the issue occurs again. But I have just tested uploading screenshots on App Store Connect again. It appears that I can upload files successfully in one go. I assume Apple has resolved the issue we were experiencing the last weekend. But anyway, it would be much better for us if Apple could at least provide a bit more explanation here.

Hi, I have tried to clear all images and load the screenshot set again and again. But the save button was grey still. So App Store basically auto-save for screenshots. It seems like any action after the upload of a set of screenshots will trigger this error. May I ask if you are experiencing the same situation now? If so, I guess it is a server-side issue.