After updating, all Nextclouds work.
One of the features was supposed to be correct URLs for Apple devices.
However: All Nextclouds show an Update (In Apps) to Calendar 1.70, yet warn that this official (!) App isn’t compatible with the Version 16!
You need to activate, with an official warning.
And, it not only doesn’t work, it doesn’t show up in the top menu.
My 2 cents