This error message occurs when the Google Calendar API servers could not respond to a Pocket Informant request within 15 seconds. This usually occurs when Google's servers are being hammered or due to issues with specific calendars on your account.
According to Google engineers the primary cause of this error is a subscription to a \"Remember the Milk\" calendar. The subscription must be completely removed from the Google Calendar settings page via your desktop browser.
We have many reports of users who have previously removed RTM calendars only to find that they are still subscribed. You may have to remove the RTM calendar multiple times within Google's Calendar Settings for it to \"stick\".
A less common reason for this error to occur is immediately after a Pocket Informant upgrade when tens of thousands of users hit the Google servers at around the same time.