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.
Just figured out that my calendars in thunderbird where not working anymore after the nextcloud 11.0.0.
A quick look and comparison I spotted the change, "caldav" changed to "dav" in the url.
In thunderbird, you can change this quickly. Stop thunderbird and edit "~/.thunderbird//prefs.js". Search for "/caldav", replace it and thats it.