I think I found a work around which is to launch the simulator first then launch Xcode then build the app. If you go the usual route your app will freeze. Also the preferences app will keep crashing at least on my system so can't sign in to test CloudKit syncing.

Had this issue a while back. I had to delete all the screenshots then log out. Log back in upload the screen shots and then it seemed to work and was able to submit my app.