My Google Nexus S running Android 4.04 (Ice Cream Sandwich) suddenlty stops syncing my Google Apps (Gmail) calendar. Only new events where syncronized with the cloud. Using the stock calendar app, no 3rd party calendar apps or widgets installed!
When my mobile was on the previous Android ICS version 4.03, I had this same issue. Solved it back then with a “factory reset”, but I don’t want to perform a phone reset now!
Clearing the Google Calendar data solved this issue for me!
Settings > Apps > All > Calendar Storage > Clear data.
And all calendar events synced like before!