Calendar broken after upgrade to High Sierra I upgraded from Yosemite to High Sierra and found that Calendar has "forgotten" all entries after the upgrade: no trace of them was to be found in ~/Libraries/Calendar. As I never activated synchronization with iCloud and also suppressed it during the upgrade, there was no direct way of recovering them.

In an effort to get the Calendar entries back, I copied the contents of the ~/Libraries/Calendar folder from my pre-upgrade Time Machine backup (overwriting some of them in the process). However, now Calendar cannot even be started, it crashes right on startup and leaves error messages like

error: (6922) I/O error for database at /Users/me/Library/Calendars/Calendar Cache.  SQLite error code:6922, 'disk I/O error'
error: Encountered exception I/O error for database at /Users/me/Library/Calendars/Calendar Cache.  SQLite error code:6922, 'disk I/O error' with userInfo {
NSFilePath = "/Users/me/Library/Calendars/Calendar Cache";
NSSQLiteErrorDomain = 6922;} while checking table name from store: &lt;NSSQLiteConnection: 0x7f971e01d160&gt;


in the logfile. What can I do to make Calendar work again and get the old entries back?

Thomas
Try this: 
Quit calendar (cmd + q), delete the calendar directory on your Mac’s HD/SSD, empty trash, reboot (with the option to open previous apps/Windows off), launch calendar, wait 5 minutes, quit calendar (cmd + q). Find the .ics files in the backup, import them with the calendar app, wait, see if your calendar works.

Another soloution may be:
Quit calendar
Find the old calendar app in your backup, and copy it to your desktop.
Copy the calendar directory from your backup, replace the current directory on your Mac. Reboot (with the option to open previous apps/Windows off).
Launch calendar from your desktop, and see if it works...