Saw this error in my iCal program and looked it up here. A lengthy post from a user in 2011 suggests that interrupting a backup routine in Snow Leopard delivered the error message, but doesn't offer how to eliminate the error or even describe the problem the error is/might be causing. I'm new to iCal, having just migrated, in November 2012, from my trusty old Palm OS, which I loved (editorial comment - Apple, why didn't you buy Claris Organizer back from HP? HP hasn't a clue. The Palm calendar, old as it is, beats the crud out of iCal).
But I digress. Does anyone have a suggestion about the iCal error? Apple? You're strangely silent on this one. After all, these ARE your support pages . . .
Ancient dual-core 19" iMac with one GB RAM running 10.6.8. Slowly.
At your mercy
iCal, Mac OS X (10.6.8), Grieving the loss of Palm Calendar