Syncing is deleting locally

I just installed the newest NC on my Raspi - no problems with it.

I’ve created a user and connected my MacBook Calendar and Contacts with it. I expected that a sync will transmit my calender and my contact data to NC - but instead they are deleted locally. What went wrong here?

I can recover my contacts and calendars locally from my backup - but I want them to be synced with NC, not deleted.

Any help is appreciated!