Not having such a great time of it with WordPress 4.0 “Benny” and HTTPS. Earlier this evening I completed a backup of my site, then deleted it entirely, and installed from scratch (version 4.0 using the Tsohost ‘one click’ application install) and reimported the posts and media. So I’ve ruled out every plugin and plugin remnant. I’ve also ruled out the theme (the redirect loop occurs on the Twenty Fourteen theme as well). Now I’m running out of ideas, so Google here I come. (Postscript: it’s now fixed)
This means the site might go up and down a bit, apologies if this happened to you. I’ll update this post to show the progress I make, and hopefully include the fix I eventually found!
It’s fixed! See full blog post (tomorrow, Thursday 11th September), but basically the fix was a bit of code added to two PHP files by Tsohost, who amazed me again with their service, fixing the problem 8 minutes after I reported it!