We have discovered an issue that occurs when a plan is shared with an ArcGIS Online group that has "shared update" privileges. When someone tries to upload an image for georeferencing in that plan they get an error message. The upload fails. Our client reported the error and we have confirmed in on our end.
The shared update group has been used so that two GIS admins are able to share responsibility of maintaining the plan.
Thanks for reporting this! We can reproduce this as well on our end. We will look into it!
Thanks, Till