Apple - Mac calendar app (iCal) displays error message "server responded with an error"

Try following sequence.

In System Preferences, Internet Accounts, deselect all Calendars to stop the sync.

Wait for a while for the process to stop.

Than click on the "Revert to Server"

Re enable Calendars.

You may need to empty your iCalendar cache. You can find detailed instructions online, but the main points are:

1. Backup your iCalendar data, particularly if you have locally stored data.
2. Delete cache files in /Users/$USER/Library/Calendars/ -- basically files with the word "Cache" in the title; there were 3 or 4 files when I did it.
3. Open iCalendar and reconnect.

Note: This persistent error seemed to start for me when I was performing a Calendar action and had a hard network failure while the action was in progress.