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: <NSSQLiteConnection: 0x7f971e01d160>
in the logfile. What can I do to make Calendar work again and get the old entries back?
Thomas