Cheating, are we? in eNewsletter

I'm having huge issues with editing newsletters in eNewsletter for http://www.milkchic.co.uk. I can't edit anything at all in Firefox (needs updating so may well be a browser issue...) as I get a "Cheating, are we?" message every time I change anything at all. I managed to get everything working in Chrome this morning, which is fully up to date. Unfortunately, every time I try to save, I get the "Cheating, are we?" message and lose all my work.

I'm desperate to get a newsletter out this week. Wordpress and all plugins are all latest versions and I'm getting the same error with only eNewsletter activated so I don't think it's a plugin conflict. Any ideas or workarounds please?