Your calendar couldn't be refreshed (Operation: CalDAVRefreshDelegateListQueueableOperation)

• Under Preferences > Accounts I unchecked Enable this account for each of my accounts
• I then closed the Calendar application completely (cmd+q)
• After re-opening Calendar, I rechecked Enable this account for each account

and everything seems to work now!

Sign out of iCloud

To fix this sign out of iCloud in the preferences pane. No data will be lost and all will be retrieved when you sign back in again. If you use iCloud Keychain, when you sign back in to iCloud follow one of the procedures described in this Support article to set it up on an additional device.

If that doesn't work

If signing out and in of iCloud doesn't work there are several other options you could try.

Re-enabling the calender account

You could try re-enabling the affected calender account. From the Calendar menu bar, select Calendar ▹ Preferences ▹ Accounts.

Choose the affected account from the list on the left and uncheck the box marked. Then check it again to enable the account.

Add the account again with the same settings

You can make a note of the settings for that acxoint, then delete it and add it again with the same settings.

Check your settings

If you can't add the account, the settings may be invalid. Verify them. How you do that depends on what kind of account it is. If it's a Google calendar and you've activated 2-step verification, you need to generate an application-specific password in your Google account and enter it in place of your old password.