Server responded with “400” to operation CalDAVAccountRefreshQueueableOperation.

Another way to fix things when you don't have errant accounts:

1. Open Calendar app.
2. Go to System Preferences > Internet Accounts.
3. Select the existing account on the left (e.g., "Google").
4. Uncheck Calendars in the list of services. Your account calendars will disappear from the Calendar app.
5. Check Calendars in the list of services. Your account calendars will reappear in the Calendar app.

Something, somewhere *** stuck. This method resets things in a sense without requiring you to remove account credentials. It's the calendar equivalent of turning it off and turning it back on again.

So how do you delete the connection to the yahoo calendar? I never made one in the first place!

Hi, I ** guessing that when you create an Internet Account with Yahoo Mail for example there's a link created between the OS and Yahoo Mail. This link includes Calendar. I'm guessing that when Apple upgraded the OS this link broke - thus the 400 error messages.

So my idea is delete the Internet Account/Yahoo Mail , restart the iMac, then add the Yahoo Mail account again.

IN MY OPINION the data resides on the Yahoo Server so I'm not compromising it. All I'm deleting is the link. Then I'm reestablishing it.

My logic is these apps are bundled together so when one piece drops I don't want to waste my time trying to fix one. Better to delete the suite of apps and start over.

This is just my opinion. I ** in no way an expert, just a user trying to fix it the easiest way. 😉

And so far it seems to work for me.

So I deleted the connection between Yahoo Calendar and Calendar. No problems now.

What's the point of all this synching if one party A updates but party B doesn't so there's a conflict?

But I don't have a yahoo mail account - so there is nothing for me to delete. I don't know why this is happening, but you are right - it seems to have happened when I did the most recent Mt. Lion update.

But I don't have a yahoo mail account - so there is nothing for me to delete. I don't know why this is happening, but you are right - it seems to have happened when I did the most recent Mt. Lion update.

First try a simple fix: back up all data, then delete the calendar account and add it back. Test. If there's no improvement, restart the computer and try again.

Yeah! I checked my ical preferences and then accounts. The WAS a yahoo account in there which I deleted!! Don't know how it *** there but now it's gone!!

Hi Christopher,

Can you please explain the process to delete the connection between Yahoo Calendar and Calendar without loosing any past, present and future data on the ICal.

Thanks,

Stephan