iCloud repeatedly deleting all my events, even after a restore

Yesterday iCloud deleted all of my calendars, and all my events (and also reminders), replacing them with two empty calendars named Home and Work. I have both Time Machine and Carbon Copy backups; I’ve restored the ~/Library/Calendars/ directory five times. Each time, my all my events are again deleted within a minute or so, unless I turn off all internet connectivity (even after disabling Refresh Calendars.)

The “Restore your calendars and events on iCloud.com” is unusable, since https://www.icloud.com/recovery/ improperly listed No Archives for Calendars, and also Bookmarks and Files. (There are ten archives listed for Contacts.)

    Sometimes after restoring, I get crash dialogs for /System/Library/PrivateFrameworks/CalendarDaemon.framework/Support/calaccessd and DataAccess.framework/Support/dataaccessd.