No problem at all. I've been on holiday recently, but my last request sent to our Moodle host seems to have fixed it!
Yes you were right regarding it being caused by a php. file missing (mod_certificate). My Moodle host uninstalled the directory of a third party plugin, but only disabled the front end part within the admin settings.
Despite being disabled, this still caused an error when using the app.
Now that it has been completely removed, it is back working again.