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.
So, I really gave it a try and invested multiple hours. Either my spirit is not strong enough today, or it is currently not possible to synchronize owncloud 8.0.2 calenders with other applications.
To calm me down, here is a list of possible problems that are the reasons I failed currently with my mission.
What did I learned?
And finally, a list of most promising howtos (always have an eye to the creation date and the owncloud version used).