This is only an issue on the Meteor/eMobile network. No other network is affected by this. The login issue has occurred on several issues only to be resolved by Meteor before recurring again. It's a Meteor network problem, not a YouTube problem.
OKay, we have localised and resolved the issue now.
Just to note that in testing we found that once the we were logged in the “Youtube not available” message might still appear from time to time when trying to open our folders e.g.: subscriptions, but from what we could see we believe this may simply be down to the Youtube servers getting a lot of traffic. If you refresh this should resolve it.