Hi everyone. I was saddened to see lots of other people experiencing this issue, but I think I have pinpointed the culprit.
I had the Item logo image for Shopify Theme File Search by EZFY app installed in my Chrome browser (which is a wonderful app that has been so helpful over the years). I have now deleted that app and now can see all my Shopify windows load nicely again.
I hope that might help someone else out there too!