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”