So I deleted "operaid_ownc2" from cPanel-->Databases-->MySQL Databases-->Current Databases and "operaid_ownc2" from cPanel-->Databases-->MySQL Databases-->Current Users.
This left no entries on the "MySQL Databases" screen (cPanel-->Databases-->MySQL Databases).
Then I removed "owncloud" from "File Manager" (cPanel-->File Manager-->/home/operaid/public_html/owncloud), electing to skip "trash" and ensuring that the "trash" was empty, just in case.
Following this, I removed ownCloud using "Installatron Applications Installer" (which was whinging about something or other at this point - most likely because I had deleted the stuff above)(cPanel-->Software-->Installatron Applications Installer-->ownCloud-->Uninstall).
Then I re-installed ownCloud using "Installatron Applications Installer" and logged-in with the "admin" (ownCloud) account I elected to create.
And it's exactly the same issue - ownCloud works fine until I click "Calendar" (which I chose to install via my ownCloud admin account), then I just get the "spinning circle" next to "Personal".
Under "Admin" of my (ownCloud) "admin" account, there's an error that reads:
* Security & setup warnings
The PHP module 'fileinfo' is missing. We strongly recommend to enable this module to get best results with mime-type detection.
* You are accessing this site via HTTP. We strongly suggest you configure your server to require using HTTPS instead as described in our security tips.
* No memory cache has been configured. To enhance your performance please configure a memcache if available. Further information can be found in our documentation.
Please double check the installation guides :arrowupper_right:, and check for any errors or warnings in the log._
When I try to update from version 18.104.22.168 to version 9.1.6 via the Updater accessible in my "admin" account, it gets to the "Updating apps" section" and says:
Extracting source into /home/operaid/publichtml/owncloud/.htituqx8hmyjyf.data/updater-data/_oc_upgrade/9.1.6_
Extraction has been failed
The Process class relies on procopen, which is not available on your PHP installation._
The ownCloud "config report" can be found here, and a fresh log can be found here.
UPDATE: Nevermind - I have managed to get an alternative solution for domain working, that is separate from ownCloud (i.e. using an unrelated software package)... It's the way I planned to have calendar synchronization originally, but it is a (mostly) superior solution.
Thanks for all your help though!