iCloud repeatedly deleting all my events, even after a restore

Another Apple bug: Trying to delete one of the calendars which failed to sync completely silently failed repeatedly. The console app recorded:

error 13:51:02.244740-0400 Calendar Exception thrown while attempting to perform a menu item's action. It has been caught in order to not leave the menu in an inconsistent state. This is a bug in the client code. *** +[NSIndexSet indexSetWithIndexesInRange:]: Range {9223372036854775807, 1} exceeds maximum index value of NSNotFound - 1

error	13:51:02.248047-0400	Calendar	(
	0   CoreFoundation                      0x00007ff8097e21ea __exceptionPreprocess + 242
	1   libobjc.A.dylib                     0x00007ff8092ad24b objc_exception_throw + 62
	2   Foundation                          0x00007ff80a8c4f01 +[NSProxy class] + 0
	3   EventKit                            0x00007ff82627dc09 -[EKCalendar reorderCalendars:betweenEarlier:later:] + 606
	4   EventKit                            0x00007ff82627d966 -[EKCalendar reorderBetweenEarlier:later:] + 131
	5   Calendar                            0x000000010bac2c55 Calendar + 2116693
…
31  AppKit                              0x00007ff80d19d085 NSApplicationMain + 803
	32  Calendar                            0x000000010b8c139c Calendar + 13212
	33  dyld                                0x00007ff8092e5530 start + 3056
)

default	13:52:28.803847-0400	Calendar	Requesting a reminders sync with reason App Activated
default	13:52:28.803909-0400	Calendar	AppDelegate: trigger throttled sync {reason: <private>}

BusyCal failed, at least not silently, but with an incorrect explanation:

You cannot delete the calendar "OS+IG" because it is your default scheduling calendar.

Deleting it from my secondary Mac worked.

• https://www.busymac.com/docs/busycal/70630-using-busycal-with-icloud#limitations
• Limits for iCloud Contacts, Calendars, Reminders, Bookmarks, and Maps - Apple Support (Published Date: January 06, 2025) Total number of calendars, events, and reminders: 50,000*

I’m now at 59,264 events on my main Mac, but that includes a duplicate calendar of 16,048 which I created during troubleshooting. Only 24,033 events have synced to my secondary Mac.

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.)

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.

Exporting from Calendar and importing in BusyCal to that calendar still missed some, 974 vs 997. BBEdit diffing sorted .ics files again revealed significant omissions, e.g. Innovation In The Era of Non-Human Intelligence 4/17/25 4:30.

Next up: Create an iCloud calendar in Calendar, and import the Calendar .ics into it from BusyCal.

Another complication:  Sometimes  Calendar has duplicate events on the origin Mac when BusyCal doesn’t, and other Mac do not.  One calendar which seemed to fail to sync completely (2088 entries on origin, 1311 one other Mac) had a lot of duplicates, which iCloud may have been smart enough to reject.  How I discovered this (which I don't recommend) was to restore it from an .ics file, waited until it synced to BusyCal, and removed duplicates there.  

    What I instead recommend is to export an apparently incompletely-synced calendar to .ics on both source and origin Calendar apps, and then in BBEdit sort and then remove duplicates; then compare with BeyondCompare.  Display diffs with context; manually scan for coherent text which appears only in the original.  Copy it, and search for it in the unsorted destination .ics in BBEdit; often the context will make it clear that it's an unimportant loss.

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”

Exporting and then importing (in Apple’s Calendar app) a medium-sized calendar (997 events) on my main Mac got most of the events synchronized to my secondary Mac (847), according to the script below, even though the earlier import mostly failed to synchronize (168 events). But the events don’t seem to show up on either the secondary Mac (even after View/Refresh Calendars) or iCloud via Safari.

on run

set output to ""

set grandTotal to 0

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

tell application "Calendar"

-- Include hidden calendars by not filtering

repeat with cal in every calendar

set calName to name of cal

set eventCount to (count of events of cal)

set grandTotal to grandTotal + eventCount

set output to output & calName & ": " & eventCount & linefeed

end repeat

-- Add total line

set output to output & "TOTAL: " & grandTotal & " events" & linefeed

end tell

return timestamp & linefeed & output

end run

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.

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

But the events don’t seem to show up on either the secondary Mac (even after View/Refresh Calendars) or iCloud via Safari.

Some events are now visible, but the number of events is still too low, 847 of 997. Exporting the two calendars as .ics, sorting the files in BBEdit, and diffing, showed that some of the omissions are significant (e.g. “10/22/10 Open Mobile and Social Computing Prof. Monica Lam”). So it’s not just that empty and duplicate items are filtered out by the export/import process. The obvious explanation is that Apple Calendar import is buggy in a way that BusyCal is not, but I don’t have enough data points to exclude coincidence.