I have recently ran into this same problem.  You might could blame it on malformed html, except the text was typed directly into the FCK Editor and was not pasted from any other source.  I have redacted any typed text as some of the information is not available for the general public but I have left anything that is code.  I did remove several <em></em> tags that were exactly that.  <em> with no content.  My user was trying to take all of the content from the second paragraph tag and convert it to a bulleted list.  When he hit the bulleted list button he had the error popup and then could no longer update, or cancel out of the edit view.  I tested on a different computer at a different station and had the same error.  All of the text was hand entered.  I entered the original though the view interface.  To get around the problem I hand entered the html code for an unordered list and was able to update.  There is a direct problem with the editor and some of the code that it is creating. Feel free to test, I plan on troubleshooting myself but have yet to get to it.  My end user used the same techniques that have always worked and are now creating an error.  We are using DNN 4.6.2 with whichever version of the FCK editor it contains.  We directly upgraded from 4.4.1 to 4.6.2. and were using the FTB until the upgrade.   

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