I too have this problem, running version 2.7.1. I think justderek may be onto something with the subdirectory issue. I have just changed the directories so that users can access from the main domain but the WordPress files are located within a subdirectory. Eg website visitors go to https://www.example.com but the WordPress install is in https://example.com/wordpress
UPDATE: Interestingly, if I just use https://example.com there’s no problem – I can preview images with no error. Unfortunately, the people using the blog want to use https://www.example.com. So nobody’s happy ??
Any help would be fab. Thanks.