We previously had a similar issue when migrating multiple accounts from MobileMe to iCloud. The issue was resolved by disabling and then enabling iCal sync through iCloud on our iMacs in System Preferences.
Always backup your calendar before troubleshooting (File > Export > iCal Archive).
The following steps are roughly from a Apple Support Community discussion.
- Open iCloud in System Preferences.
- Switch off (uncheck) iCal sync in iCloud System Preferences pane.
Backup iCal before doing this. You will need to remove calendar data.
- Open iCal and verify the iCloud account is removed in the iCal preferences under accounts.
- Close iCal, reopen System Preference and switch on (check) iCal sync in iCloud System Preferences pane.
- Reopen iCal and verify the iCloud account is enabled in the iCal preferences under accounts.
At this point if the problem persists you might try completely signing out of iCloud and then signing back in. But remember to backup all related data (iCal, Contacts, Mail, Photostream, etc) before turning off iCloud sync!