My only advice would be (and you may have already heard this, but it's the extent of my knowledge) is in order to avoid being unable to save/submit, make sure there's no "profanity" or copyright material in your team's name, nickname, and abbreviation. Which can be kind of hard since EA considers a lot of pretty silly things to be "profanity," for example I was trying to recreate Miami's TE Riley Williams, and it wouldn't let me because the name "Riley" was considered profanity. So as long as your names check out, its possible you have yet to select a color for your jersey's/helmets. If you're using the default colors and trying to save, sometimes the colors it starts you off with won't actually be selected so you have to go select them yourself in order to save. If all that is in order you should (in theory) be able to save, and not have to deal with the 400 message. As for uploading images/logos, just make sure they are in the "PNG" format, and preferably size small. I tried to mistakenly upload a JPEG thinking it would work and I sat there stuck on the loading screen for like half an hour before realizing.
