I don't thing nextcloud 12 is the issue here. Simple the old used "caldav" application and the connected AndroidCaldavSyncAdapater is not working anymore. I've removed the users everywhere as well as the application. I've switched the phone to use the DAVDroid installable via F-Droid.
The setup is so easy I won't write a howto section here.
Just be fair and donate some money to DAVDroid since the application is working out of the box.