Today I was playing with CalDavSynchronizer. I tested different configuration options for synchronization and ended up with some of my calendar entries deleted. I knew there were a couple of other Nextcloud users on my server so I checked whether it is suitable to restore the calendar from last night's backup.
This is written based on my own experience so YMMV. This has been tested on Nextcloud 16.0.3 and MariaDB 10.4.6.