New Calendar event entries instantly revert back to New Event

Boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and reboot normally. Run and check to see if the problem persists.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

bwadma wrote:

This problem was widespread some time ago (maybe 4 years ago), but for me it re-appeared in the lest weeks. I am not exactly sure since when, maybe together with the update to macOS 11.4.

It does not happen each time, but quite often:

A new entry is created in the Calendar.app, it is named with some text. Shortly after, the event entry is there, but the text is gone an replaced by "New Event".

Any idea what the cause is?

Has the further updates to Big Sur been applied ?? Current version os Big Sur 11.5.2 and brings Bug Fixes and Security Fixes.

Strongly suggest A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. 

🤞