Calendar keeps popping up with errors and I cannot clear it?

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Please take each of the following steps that you haven't already tried. Stop when the problem is resolved. Back up all data before making any changes.

Step 1

In the Internet Accounts preference pane, delete any calendar accounts you don't use.

If there's no improvement, delete all calendar accounts and add them back. With an iCloud account, all you have to do is uncheck and recheck the box marked Calendars & Reminders in the iCloud preference pane.

Step 2

You may be trying to access an invited event that you declined, or one that was rescheduled after you accepted it. The error message should give you a clue as to what it is. If the calendar is on iCloud, you can bring up the Calendar on the website and select

Show Declined Events

from the popup menu with the gear icon in the upper right corner of the page. The declined events will appear in gray, and one of them should match what's shown the error message. [Credit for this solution to ASC member Psi.]

Step 3

If you get the error when trying to accept an invitation in an email attachment, it's because the organizer didn't properly create the event.

Step 4

Follow the instructions in this support article. Modify as necessary for a calendar that's not on iCloud.