Thanks for your interest. I still haven’t figured out the problem but have learnt a few things.
I use the same theme and Calendar Pro (all same versions) on another of our websites and I have no problems so I’m guessing it’s not a theme conflict. I can’t really change to Twenty16 because we don’t have a development site, just a live one.
I have not had problems navigating the front end of the site, just the fatal error when I go to edit a page.
Today’s error message is slightly different from last week’s, but still a Tribe error: the-events-calendar/src/Tribe/Asset/Factory.php on line 3
Deactivating event calendar pro resolves the problem. But today I discovered that de-activating a DIFFERENT plugin (fyi: team booking) also resolves the problem! It’s an up-to-date plugin but not one I’m really using anymore so I’ve deactivated it and can now edit pages AND can use calendar pro.
Problem isn’t really solved/figured out but at least I’m able to now get done what I need to!