I know it is an old question, but maybe I noticed something that it might help you.
Today when I try to login to Itunes Connect via browser, Apple returns me a message "Your Apple ID or password was entered incorrectly." , but I'm sure that my id and pass was correct . It seems apple side problem, not ours.
I think that there is something that it doesn't work in Apple, and it is not an Xcode bug.
I don't know why they don't warn anyone about that.
EDIT
Now Apple updated its System Status - Apple Developer.