YouTube have recently switched to their API Version 3, and subsequently there were changes made to the URLs of the YouTube RSS feeds.
Previously you were able to enter a URL such as this one:
…that would in turn be converted into something like this
…however these feeds no longer work as they work with Version 2, that’s no longer in use.
For this reason, YouTube feeds must now be changed to the following:
where all you have to do is change the Username or Channel ID to the one of your choice.