This error occurs when Google stops a request from Pocket Informant and returns with a "Could not generate feed." error. The Google servers are designed to error out after 15 seconds of processing. Google's servers can get overloaded when there are too many people accessing the servers for example after an update. If you have tried again and again at different times of day and continue to get this error then you may be seeing an issue with the calendar data you have being too processing intensive for the Google API servers.
If this is the case then you may need to look at the calendars you are syncing and unsubscribe from some such as holiday, week number, temperatures. These calendars have been shown to cause issues with Google API servers from time to time. "Remember the Milk" calendars are other calendars that cause these issues. It is important to unsubscribe, not just deactivate these calendars.
Another option would be to create a new Google Calendar account for these subscribed calendars and have Pocket Informant sync with that account as well as the main one.
In general all of these errors are caused by Google's servers not being able to handle the load and while it may correct itself after a days on its own, anything you can do to help Google's servers manage the load will help you sync more smoothly.
You can also try logging into your Google account on your desktop computer and typing in: http://www.google.com/calendar/feeds/default/allcalendars/full and seeing what Google responds with.