The google calendar sync had been working fine for a long time now suddenly starting yesterday we started getting the following error. I saw that this has happened in the past and google engineers (SRE) had to apply a patch to fix this.
com.google.gdata.util.AuthenticationException: Unknown authorization header
Google Account Authentication APIs (ClientLogin, AuthSub and OAuth 1.0) are deprecated as per deprecation policy. http://googledevelopers.blogspot.com/2012/04/changes-to-deprecation-policies-and-api.html
Important: AuthSub has been officially deprecated as of April 20, 2012. It will continue to work as per our deprecation policy, but we encourage you to migrate to OAuth 2.0 as soon as possible.