Hi, I would like to add a calender from another nextcloud installation (office) to my privat nextcloud-installation. Is this possible?
You can download your existing calendar by going into the web interface, then the calendar app. Click the 3 dots next to the calendar you want to download, and select the download option to save the calendar.
Go to your personal server, and then into the calendar app, at the bottom left you should have a gear symbol, which says ‘Settings and Import’, click this and then select the import option and choose the .ics files that you downloaded.
@ @jonnie_r: This is just an export and import.
I think @LinuxHeld wanted to access to the calendar from one NC to another NC. As federated file sharing is working.
Yes it is just export and import. I have obviously misunderstood his requirement 
You can share a calendar publicly, copy the “iCal feed” url and add that as a subscription in the other instance.
Sharing among different instances with read-only and read-write support is also planned. see https://github.com/nextcloud/server/issues/1440