Have you setup a Google + account ? ?
UTube requires it now....
Does it? I don't have a G+ account and can still use Youtube and see comments as usual.
I'm basing my assumption that I don't have a G+ account based on the fact that on the Google home page, when logged in, I see a "+You" link which, when clicked, directs me to "Join Google+ by creating your public profile". I've never knowingly signed up to G+ and don't intend to.
I deleted my Youtube account for this reason.
Ironic