iCloud Calendar 503 error deleting entries after .ics import

For years I've synced my Google calendar with my Apple devices, and wanted to move my calendar from Google to iCloud. I imported an .ics file successfully to Calendar on my MacBook Pro (16 years of entries, but still only 7MB), and waited for the other devices to sync.

Every time I have tried this, after an hour or so I get a '503' error message saying that a particular entry couldn't be refreshed (it's a different entry each time) and around 70% of the entire calendar is deleted from all devices. Can anyone explain what's going on here, and whether I can get round it?

MacBook Pro M2 16GB MacOS 26.5.1, iPad Air 11" M2 iPadOS 26.5, iPhone 16 iOS 26.5