In my testing, the issue is happening because of an image-type file upload detail. The underscores that are created as part of my phone's image naming scheme were causing this error. I also found that spaces caused this issue, so there's probably many others as well.
To address this for a public-facing form, I used the bind::esri:parameters field to give the photo being uploaded a default name of "img". You can see the details here and my syntax below. You could be fancier and use info from your form, but I didn't need that; be careful of special character issues if you do.
filename="img"