Refer: https://michaelkummer.com/technology/fix-calendar-cant-save-event-x-exchange/
The Calendar app often gives you three options: Revert to Server, Try Again and Ignore. In some cases, neither option resolves the issue, and the error message keeps coming back, every time Calendar tries to synchronize with the Exchange Server.
The solution involves deleting some cache files and restarting all Calendar processes. In a nutshell, you have to:
The following 3 files were deleted and fixed the problem:
- Go to ~/Library/Calendars
- Calendar Cache-wal
- Calendar Cache-shm
- Calendar Cache
- Quit Calendar application (Command + Q)
- Quit Apple Mail
- Open Activity Monitor (through Spotlight or Launchpad)
- Search for “Calendar” and quit all the relevant processes
- Open Finder and navigate to ~/Library/Calendars
- Double-check Activity Monitor to make sure no Calendar-related processes are running
- Delete cache files
- Relaunch Calendar