I have just upgraded from Snow Leopard to Mavericks on a 2009 vintage MacBookPro, and have the same issue as the two contributors above.
When I import a calender with my schedule in .ics format to iCalender, the cloud works fine with all of my devices.
Then when I import a similar .ics file with another schedule to iCalender, the repeating error messages as reported above appear, with the logjam reported by pberke.
I have tried lots of troubleshooting. I've deleted all of my calenders from my macbook and iCloud and started again and still the problem occurs when the second calender is introduced to the mix.
Other calenders which I have created manually do not seem to cause any problems.
I would be grateful for a solution, or could you please put the synch via iTunes option back as that is the way I used to do it before in Snow Leopard.
Regards,
P