I have several test cases now that cause an error when uploading files to AGO's utility.arcgis.com services. These are simple JPG files, usually less than a couple hundred KB in size. Quite small. The resulting error is:
As I mentioned, this happens with services behind AGO's internal proxy - utility.arcgis.com. Meaning, I have added a feature layer to AGO from an arcgis server, and used AGO to store the credentials to the service. As a result, this feature service gets a new URL pointing to a proxy inside of AGO. This is where the bug occurs but the bug cascades into Survey123 as well when this happens.
The related bug is https://my.esri.com/#/support/bugs/BUG-000107195
It would be great if someone at esri could take a look at this and fix the issue. As a result of this, the survey becomes "locked" meaning the survey appears in the user's outbox but it will not allow the user to modify it in any way. So it doesn't even matter if I can find the image that's causing the issue because the user cannot remove the image from their survey and resubmit - the survey gets stuck in this limbo and the data collected is ultimately worthless.
Here are some images that fail to upload (attached).