I found an issue with the FCK Editor, that I thought I should post about incase others run into this.  The fck editor doesn't seem to cope well with badly formed html.  This is understandable but rather than informing you of the problem you get a page error stating 'Invalid Argument' then a reference to the fckeditor.html page.

To reproduce this error in DNN 4.5.1 have a font tag overlap a p tag. 

Eg; add an HTML/Text module to a page and add the following:

<P align=center>
 <FONT face=Tahoma>
  <B>Badly formed html with a font on the outsite of a paragraph</B>
  <BR>
</P>
</FONT>

Notice that the font tag should be closed before the end p tag but isn't.  Obviously this is bad html, but it happens sometimes.  This was an issue because we recently upgraded from a site where the FTB editor allowed this.   After the update this page could no longer be edited.

Temporary Fix:  Correct your html.  If you need to you can load and save it directly in the database by calling GetHtmlText [moduleid] and UpdateHtmlText [moduleid] 
(Alternatively copy and paste, and delete the module, then add it again)

I'm experiencing the exact same problem. Only in IE actually.  I'm using the same code as above as an example of code that breaks it. Once the code is in the source, I can't go back and change the source code... I can't clean up the code using the editor.. or save the contents. This works fine with FireFox.. but it would be really great if there was a resolution for this in IE.. Any ideas? The error references /editor/fckeditor.html as the code in which the error occurred.

Screen shot: