Today I changed from Quick Cache to W3 Total Cache on my site, and eNewsletter kept giving me a "cheatin' uh" error during editing a newsletter. Totally disabiling W3 Total Cache has fixed it. I can disable when I'm composing I guess. But is this a known issue? If not, perhaps you can put it on your list of things to check. It seems like this is a very popular caching plugin and might affect more people than me over time.