Thinking everything over, the most likely differences between one of our Discourse installations and the vanilla sandbox would be in the image upload settings. I didn’t have time to rebuild the app, but don’t forget there’s also the client_max_body_size setting in app.yml. There must be some interaction between that or maybe the specific Nginx version in the Discourse container with the iOS client. I get the impression that the client falsely thinks it’s receiving an error, whereas according to my testing the server is happy to accept large image uploads.