eNewsletter content not showing

I am unable to create any content for my newsletters, using the Content editor. When I type anything, it does not show up in the preview pane on the right. When I save the newsletter, and re-open the Content editor, all my text has gone.

I can successfully create new settings, background images, even send previews of my "blank" newsletter, etc, BUT none of what I do will allow me to add content to my newsletter!

I suspect some plugin conflicts, but I just don't know where to start. HELP!