Problem with Calendar on iMac not syncing with iPhone and iPad
Catalina OS iMac late 2013
Error message "your calendar could not be refreshed"
The server responded with
“400”
to operation CalDAVWriteEntityQueueableOperation.
iMac 21.5″, macOS 10.15
"Problem with Calendar on iMac not syncing with iPhone and iPad: [...]The server responded with'400'[...]"
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Restart your Devices:
There is an error connecting to Apple's Server --somehow the connection is corrupted. So, save what need be, and then restart your devices.
Restart your Mac
Force Restart your iPhone:
Go Here: Force Restart your iPhone - Apple Support
Restart your iPad:
Go Here: Restart your iPad - Apple Support