While there is no official confirmation yet from Google, it appears that the Google Calendar API is currently down. I am seeing CloudPull have difficulty backing up Google Calendars, this link returns a server-side error, and I am reading other reports of Google Calendar backup problems.
I will update this post as I learn more. In the meantime, you may see difficulty backing up Google Calendars over the next few hours.
Update 11:25 PM Boston time (EDT): The problem appears to be resolved.
Update May 2, 2013: Google has since acknowledged and apologized for the outage.