I'm having similar trouble on DNN 4.7.0, but FCK Editor rejects only certain kinds of content.

The most frequent errors occur when I try to insert an image that has been uploaded to DNN. If I link to an external image (Photobucket, etc.) then the content updates just fine.

Turning off AJAX has no effect.

Edit: Just a thought, and probably not relevant. This occurs when I use IE6, IE7 and the latest build of Firefox 2. I'm guessing it has nothing to do with the browser...

Steve Aunan wrote Decided to try a few other settings, and found one that worked. Changed "Page State Persistence" from "Memory" to "Page" and now FCK Editor works properly. I turned AJAX back on without a problem...

This is the only thing that worked for me.  I went from barely being able to use the FCKEditor in any module to it working normally again when I changed that setting.  I know I'm taking a hit on page weight but the alternative wasn't acceptable.

 Sebastian that's a good point. 

I was having the same problem in CHROME - switch to Firefox and the Rich Text Editor works beautifully.