I imagine you're running into the "potentially dangerous request" error. This is because of the characters submitted by the fckeditor.
If you
Google the error[
^] there's a ton of information out there.
Unfortunely, the fix isn't the same for different versions of .Net and you haven't specified what you're developing on. You should be able to figure it out from the above query, otherwise, go ahead and ask a follow-up question!
Cheers.