iCal/Calendar and GMail - constantly errors and problems
The last post on the first page of this discussion states it is a known Google problem. (I haven't personally verified its accuracy, though).

Quoting @Kjell-o:


  Just so everyone is aware. The issue is indeed in Google's system, not in iCal. They are working on a solution. I received this directly from their enterprise support. If you look at the calendar logs, you'll see a ton of error for the calendar service - I'm getting ones that say, premature EOF.
  
  On another note, the google forums say that the shared calendars no longer act as "delegates" but as normal calendars and as a result they show up in the regular list. This may be the "duplicates" that some people are receiving. If this is you, uncheck the delegates as those will be superflous and most likely the cause of the 404 messages.