Fortunately, with the upgrade to 1.1 we now have shell access

In the nginx error log (/var/log/nginx/error.log) I found errors of the form:
FastCGI sent in stderr: "PHP message: PHP Fatal error: Call to undefined method Sabre\VObject\Property::getValue() in /usr/share/owncloud/apps/calendar/lib/sabre/backend.php on line 310"...
A Google search revealed the problem lies in Owncloud's code:
https://github.com/owncloud/calendar/issues/551
and the one line fix:
https://github.com/owncloud/calendar/co ... f4aedd1f5d
I edited /usr/share/owncloud/apps/calendar/lib/sabre/backend.php on my opi and it solved the problem.
For those of you who don't want to mess around with vi and the command line, OP will release a fix in due course
