I'm having the same issue - same error message. It's occurring in IE7 & IE8 but works in Firefox. I have Version 5.4.2.1.

I've narrowed my issue down to the HTML Block that I have on a specific page, which contains a scrolling marquee. The marquee works but I can't update the text without getting the error message when Updating. Also not able to update the other text block on that same page without getting the error message. There are no problems updating text blocks on any other pages. Not sure now if it's the HTML block or the scrolling marquee.

It's the HTML block. Tried other HTML code, no marquee, and still can't update HTML block. Must be a bug. It was working just fine until a couple weeks ago. Any one else having this problem?