Here is what I learned: Synchronisation (CalDAV & CardDAV) from Owncloud to IOS 10 (maybe even 11) works just fine without SSL (https), but in IOS 12 it does not. But if the connection supports SSL, it works just fine. What is a bit misleading on Apples part here, that even in IOS 12 it lets you choose between unencrypted and encrypted, without informing that the first just won’t work.
What I use: OC 10.0.9.5 on a Strato hosted server.
I entered as credentials in IOS just the domain to where Owncloud is located, not the full path, even though it says so in the out of date User Manual here:
So if I just use as server: “example.com” IOS will find the full path by itself, IF that domain is being redirected to the folder in which OC in located in. It did NOT work with “example.com/owncloud”
And remember, it is important that there is a working SSL certificate assigned to the domain “example.com”.