For anyone else who runs into this, the answer is either "wait three or four days" or "upload on hotel wifi". I'll need to get home before I find out which.

I have this issue too. Xcode won't log into my account correctly in System Preferences (aI dialogue pops up with "Unexpected nil property at path: 'Actor/relationships/providerId'") so it can't possibly be problem with my code signing settings. I can log into all the Apple websites and portals I've tried, so it's not my account. I have an update I'd like to submit to the store, and I can't at the moment.