This is still an issue in 2019. I was making a Story Map Tour, and I am unable to share my progress with my colleagues, since it will not save changes. The map data is hosted on AGOL, while the images are hosted on Amazon S3.
In general, I would lobby for more useful error codes. Perhaps my photos are still to large? I guess I'll try re-uploading smaller versions. But will they be small enough? Who knows? What is the ideal size? Maybe the photos aren't the problem, but somebody on Stack Exchange found that to be their issue. But regardless, Esri's fail codes are notoriously useless.
I'll be sure to update if that happens to work.
