Hey Scott & all affected - appreciate all the input on this since the original post, strangely reassuring to know this is a problem for a lot of people and not just me.
I'm still getting this issue despite trying all the fixes. AND PLEASE, don't post the macros fix again, I'm getting sick of seeing it - IT DOESN'T REMEDY THIS ISSUE!
I have noticed that the issue seems to only happen once with each file - although this could perhaps be only over a certain timeframe. For example, I opened 4 word documents late last week - they were in a file that contained 10 documents in total. Today
(4 days later), I tried to open all these 10 documents at once, but only the 4 that I had previously opened actually opened - with the rest showing the error from my original post.
Although I have no evidence to back this up - I do find that it's older files that I'm opening that seem to get this issue. For instance, docs I created last week are opening fine - but the docs I'm getting errors from I created in Sept 18. Again, restarts
of my Mac are still working, but this is still enormously frustrating.
Twitter support points me at the helpline, but I'm loathe to waste my time if other people aren't getting anywhere.