iCloud repeatedly deleting all my events, even after a restore

After I turned off iCloud synchronization for Calendar, my Calendar data gets deleted more slowly, after a couple of minutes. My Mac won’t sign out of iCloud; I just get a Spinning Cursor Of Pain next to “Signing Out.” I wasn’t allowed to change my iCloud password on my Mac; after I changed it on my iPhone, my Mac still thinks it‘s signed in, and I still get the Cursor Of Pain when I try to sign out. My Calendar data still gets deleted a couple of minutes after restoring via the Finder.

    I can’t back up my Calendar; File/Export/Calendar Archive silently quits after an implausibly brief time, but the resultant .icbu file is too small by a couple of orders of magnitude.

    I would like to think that Apple Quality Assurance deteriorated after I left for the second time, but the rot actually started the year I rejoined it.

Today’s Data Loss Update:

Upgrading to the latest version of MacOS 14 made no difference; updating to 15.4 24E248 made things worse. Restoring ~/Library/Calendars/ no longer restores Calendar data; it now seems to reside in ~/Library/Group Containers/group.com.apple.calendar/. My only option seems to be restoring a couple of dozen individual .ics files I backed up in one of the intervals before my data got re-deleted. (Not sure what to do after that to restore my Reminders.)

For Reminders, the Apple app seems useless, but fortunately my Beehive/Guided 2Do had a good backup.  I’m restoring data now with the Replace Server Data option, which is slow but seems to be working.  (Tip:  To see progress, use the Network option in Activity Monitor.)

Once I signed into iCloud again and enabled Calendar sync, I started getting server errors for many of my calendars, e.g. :

Your calendar events couldn’t be refreshed.

The calendar “Academe” failed to update because of an unexpected error.

The request for “Academe” in account “iCloud” failed.

The server responded with

“503”

Sometime before 2025-04-18, the iCloud Calendar web seemed finally to get updated, though my devices were obviously missing some events. (Around the same time the Contacts app on my main machine stopped updating, though the local BusyContacts did, as did the web app.) After restoring with 2Do, the Reminders web app seemed to get all the categories, but none of the events.

My sacrificial laptop got only 3174 events, out of 34265 events. (Counted with a script written to count events by DeepSeek, replacing an old one broken by API changes a decade ago.)

on run

set timestamp to short date string of (current date) & " " & time string of (current date)

tell application "Calendar"

set totalEvents to count of (every event of every calendar)

end tell

return timestamp & ": " & totalEvents & " events"

end run

I seem to be having some luck after exporting my biggest calendar from the Apple Calendar app on my main Mac, and then importing it into BusyCal, then moving the imported copy to iCloud. (I did get a warning that this loses invitees.) Some are now showing up on my secondary Mac and iCloud on the web. One warning apparently from BusyCal on my main iPhone, that an event had been modified on server.