Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
Jay, this error is likely due to a caching problem on the iMac.
To fix it,
3. Open Calendar and wait for it to synchronize.
This will take care of most problems that are to do with a caching issue.
I'm not finding these caches. On the hard drive I have Library/Caches/ and then lots of options but I don't see Calendars or com.apple.iCal
Are you still there?
I found the Calendar Cache and sent to trash. Still can't find the apple.iCal cache
Can you search for apple.iCal cache in Finder?