I am entering the password correctly. I am able to log in to both me.com and iCloud.com. I tried @me.com and @whatever_is_my_apple_id_domain_is - there are no problems with the password.
I tried changing my password, but this did not help (supporting that this is not a password issue). And yet, I can't login.
What should I do to be able to log in to MobileMe without Unknown Validation Error?

