How I solved it:
Looked at the network console in my Chrome, and looked at the responses of /appScreenshotSets api calls. It will make a call each time you switch locale. Inspect each of them. For each, there will be list of screenshots, and look for the one that does not say "state": "COMPLETED". Look at other information there, and you will be able to figure out which screenshot it is (for example, your original filename will also be there). Delete and reupload that (way few seconds after upload before you navigate away).
In my case one of the screenshot said "UPLOAD_COMPLETED" instead of "COMPLETED". Reuploaded that, and it app review went through.
If you want to narrow your search, look at the response of the api call that is made when you click "Add to review", it will give you a uuid for the exact screenshot(s) that failed. Now search for that in the appScreenshotSets api responses.
Cause of this (For the apple engineers):
You guys have designed screenshot upload to be a 2 step client driven process. Essentially, client first uploads an image, and then later has to make another API call to PUT an image somewhere, but the UI already shows success after the first step itself, and user navigates away, essentially failing the 2nd call.
Either make the second call a server side step, or make the UI show upload success only after both steps are done.
We are having the same problem. The message that we receive is: Unable to Submit for Review The items below are required to start the review process: There are still screenshot uploads in progress. We have converted the images from jpg to png - no difference. We have deleted the images that that said image load error - no difference We have tried loading on different computers - no difference We have never had this problem before At first the images all show as having been loaded. There are no errors. Then we click submit for review, and all the images are greyed, and say that they are still uploading and are being processed. Some get the image loading error. When delete all the images and reload, it is not the same images that fail the image load test. This gives the impression that Apple does not value our time. There are 10 pages of people who have had this problem going back for over a year. This is unacceptable.
This is a server side problem and it is an embarrassment for apple not to even acknowledge the problem let alone fixing it. All developer-facing tools are hostile and issues filed using feedback assistant are completely ignored.
Please look each screenshots you uploaded, I found one and re-uploaded. It worked! Thanks!