Cannot edit page: white text on white background

I cannot edit one page on my site
http://www.feedbackreigns.net/solutions/evaluate-solar-panels/

When I press edit button I get a screen with white text on a white background.
When I select all (control A) I can see the text
Same happens for text and html views of the page

Also, when this white text on white page appears, the control buttons for adding links and spelling check etc do not appear.

I can edit other pages and posts.

I had this same difficulty with what was a long post
http://www.feedbackreigns.net/economics/step-by-step-guide-for-working-out-whether-getting-solar-panels-is-good-financially/

I copied this problem “post” and pasted it into a new page. (I removed about 5 lines of html code that related only to the post from top of the file – and the corresponding 5 lines of html code from the bottom.)

As described above, this page based on the old post is now giving me this problem.

The problem goes away sometimes. This was how I was able to change my long post to a redirect to the page – which is now a problem too.

I have seen suggestions about deactivating plugins – but I love all my plugins.

I have tried “emptying cache” in Firefox

I tried Pages > All pages > “Remove from word-fence cache”. This seemed to resolve the problem for one edit only. When I tried this again – no joy

WordPress 4.3.1
Twenty Fourteen theme