I’ve replicated the issue. Relief. Ok, I’m offline tomorrow but will come back to you on Friday with a solution. With Page Builder activated the Posts Page works.
Ok, I see the issue. It’s happening in PHP versions above 7.1.6, so if change my local server down to 7.0.20, there is no issue. I’ll hopefully have a fix out for this on Monday. Thanks for all of your help and sorry for the hassle.