I'm having this issue on a dev environment and a quality environment, all of a sudden. The issue is that whenever I edit a page, imagine to change a title or add a webpart to the rich text html field, through any internet browser, the whole content of the page disappears. My logs show nothing relevant and this is stealing my sleep hours. Any idea or can I provide you any informating that may help you help me out? Thank you so much in advance!
Somebody else seems to have encountered a similar problem. Like Tiago Duarte mentioned, I would first check the webparts you're using on the page.
Add "?contents=1" to the end of your page URL, then close the web parts one-by-one.
After you close one, check whether editing works normally. If not, close another and test again.