divlaine in application loader did you login with special public 16 chars key? (It's created in appleid.apple.com)
I ran into this the other day when I was uploading an app. I was trying to log in to Application loader with my Apple ID password, but, as divlaine said, I needed to use an app-specific password. There's more info on them here: https://support.apple.com/en-gb/HT204397
It did strike me as a bit odd that Application Loader is treated in the same way as third-party apps, but there we go!