Since today i could not upload my apps to MAS. I use “Application Loader 3.6” and 3.7 (from Xcode 9). Always get the error message:
Please sign in with an app-specific password. You can create on at appleid.apple.com
Maybe two factor authentication problem?
It is the two factor authentication. Disable it applied.apple.com and “Application Loader” works again.
When the older ?TV generations didn’t get updated to support 2FA, the secret was to put the 2FA code at the end of your password and submit again. I wonder if that trick works?
NO! DO NOT DEACTIVATE 2FA, PLEASE! 
Just create an App Specific Password like Apple suggests and login with this password. Works just fine and your account is still a bit more secure…