Hi Rebecca, yes I've definitely had that: it works for editing one or two pages with those reserved words, then it just won't go.
Try adding https:// to the beginning of the url for now.
I know that it works and saves without a problem (you will just have to navigate the page without any theme applied to it, which can take a bit of getting used to!). Until I can find out what's causing it, at least you will be able to save your pages!
I had one suggestion that it works fine through HTTPS because it encrypts the information. Whatever stops it going through HTTP obviously doesn't have a problem once the text is encrypted.
Any ideas from anyone reading this would be appreciated!