I found this old tutorial that add a

--template webdav

to the #Add remote database part but it didn't solve my issue

https://itectec.com/ubuntu/ubuntu-how-to-sync-contacts-and-the-calendar-on-ubuntu-touch-with-owncloud/

I have tried also to contribute to this script : https://github.com/UT-ilities/UTouch_CalDAV_and_CardDAV_synchronization
but it isn't working neither.

I have spent hours trying to make something working without success. Too bad... I'll wait for some good news to come then or some help on how to make such script working on UT 16.04 with nextcloud.