Can’t see text in editor

For this page and all others I suddenly can’t see the text in the editor whether in visual or text mode. It’s there but I can’t see it, which makes changes quite difficult!
All I see is white space.

Everything is up to date and this does not happen on my other WP sites with the same host.

Any ideas what’s causing this?

The page I need help with: [log in to see the link]