Changes in shared Calendar results in no Notifications [iOS]

I’ve created a calendar on Nextcloud 14. I’ve successfully shared this calendar with User2.
On two iPhones with iOS 12.0.1 on it, I’ve successfully added the calendar, following this tutorial:
iOS - Synchronize iPhone/iPad
The problem now is, that if User2 makes a change in the calendar, I don’t get a notification. It’s the same the other way around.

Before we used a shared calendar on iCloud and every change in the calendar resulted in a notification on the iPhone.

Any suggestions about how make the changes visible again via notifications on the iPhones?

Thank you,