I found an old Nexus 5x and installed the linageos-fork from the e.foundation on it.
All is working fine except some WiFi-Access Points where the Phone connects itself to disconnect itself right afterwards and tries the same game again.
Long story short, thanks to reddit.com, the answer is to enable Bluetooth.
With Bluetooth turned on, the wifi connection is table.
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).
After a bit of a slow start, Android’s application ecosystem has proven to be versatile and very developer-friendly. You are free to develop an app for Android and publish it to the Play Store with minimal restrictions. This has led to a plethora of really cool Android apps, some of which aren’t available on iOS or other platforms. Running Android apps usually requires an Android smartphone or tablet — obviously! — but what if you currently use iOS or another mobile OS, and want to try out Android without actually getting an Android device?
To bad there is no working solution for linux out there, so you need to "wine things up" or use a vm.