I'm also suffering from login problems, so I researched it a little.

I have an old youtube account A and an old gmail account A@gmail.com. they were originally not connected. I can't log in with that account regardless if it's connected or not. I tried all possible combinations with usernames and A's youtube password and A@gmail.com's password. Nothing works.

I registered a new youtube account, B with the email C@asdf.com that account worked fine in cutetube.

Then I created a new youtube account D from an existing gmail account, E@gmail.com. This one also works.

Now here's the spooky part:
I disconnected A from A@gmail.com, and changed the email for A to C@asdf.com. First I couldn't log in, but when I changed the password for A to the same as B's password, suddenly I could log in as A. If I change the password to something else I can't login anymore.

So that's my take is that if you have an old youtube account with a password and connect it to a google account with a different password, it won't work. As with most things in life, I blame google.

(come to think of it I haven't tried disconnecting A, changeing password to same as A@gmail.com and then reconnecting. That might work. I'll get back to you on that)

the app is very nice btw.

Edit: Yusss! It worked. So if you're having trouble logging in, one solution might be to unlink your youtube and google account, change the password to the same as you google account and then relink the youtube and google accounts. At least that worked for me.