I'm also having this problem when trying to modify the app preview video. My app has been waiting for a few days now and we're on a tight schedule so we don't want to remove and re-add the submission, but I would if I could rather than wait for a possible fix. Once it's "In Review" those app details are locked!
I wouldn't advise trying to mess with this, but just for debugging I inspected the webpage data. Tracing through the code it looks like it's bailing out of allowing dropped files in the "drop zone" (where you upload images/videos) if the screenshots are set to "isEditable = false". The short story is that images are locked right now and that appears to be set by the Apple server on the app, rather than being a client-side webpage bug.
Please fix this! The wording on all the FAQs and help pages I could find state that screenshots (and app preview video) should be editable while waiting for review.