When opening the Calendar app, I am greeted with a box that says "This calendar can not be refreshed." After clicking on "OK" another box pops up saying "The server responded with an error." How can I get rid of these?

Question

When opening the Calendar app, I am greeted with a box that says "This calendar can not be refreshed," and after clicking ok there is another box saying "The server responded with an error."  How do I get rid of this.  The calendar functions all right, but just have to close both of those boxes after doing any action in the calendar app.

Answer 1

Please take each of the following steps that you haven't already tried, until the problem is resolved. Back up all databefore making any changes.
Step 1
From the Calendar menu bar, select
Calendar ▹ Preferences ▹ Accounts
Choose the affected account from the list on the left and uncheck the box marked
Enable this account
Then check the box again. Test.
Step 2
Make a note of the account settings, then delete the account and add it back with the same settings.
Step 3
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.
Step 4
Quit Calendar. Triple-click the line below on this page to select it:  
~/Library/Calendars/Calendar Cache  
Right-click or control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu.* A Finder window should open with a file named "Calendar Cache" selected.
Move the selected file to the Trash. There may be one or two other files in the same folder with names that begin in "Calendar Cache". If so, delete those files too.
*If you don't see the contextual menu item, copy the selected text to the Clipboard (command-C). In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.