How to log in with the native YouTube app?

Question

I created a new YouTube account to use functions like "watch later" on my iPhone4, my PC and my AppleTV2. But if I try to log in with this account on my iPhone and the native YouTube app, I always get the error message that the username and/or the password are wrong. I used as username the email address I used to create the YouTube/Google account and which I use to log in the web interface.

What I'm missing? I'm sure that the credentials I use are correct.