I have something similar. But when I click on Revert to Server, it goes away and immediately comes back with the same alert. If I click on Go off line, the alert dissappears, but a short while later it returns with the original alert. This is a pain because I am off line most of the time, or busy getting the icon ical in the dock to stop jumping.
So I reentered the event using different wording and deleted the original event. All that happened was the alert returned but using a different alert.
Sorry not a lot of help.
Don't know how you must have solved it, But mine problen now appears solved. I am not exactly sure how mine was finally solved because I tried several methods.The one I think did it, was to close iCal, delete the tick against iCal in iCloud then reopen iCal, then reactivate iCal in iCloud. Unfortunately I don't remember where I got it from
so cannot credit that persons comments