ICal error message - 'revert to server / try again / ignore'

iCal error message; 'revert to server / try again / ignore' pop-up happens whenever I add an iCal entry. I have this syncing with google calendar - which seems to be working both ways, but the error message is disconcerting and pressing any option does not remove it. The only way is to shut-down iCal. Any ideas how to fix this please? TIA!