We've been plagued by this problem since we upgraded from DNN 3.3.7 to DNN 4.8.0 (and now to 4.8.2).  We like the new FCK Editor better than the old editor from DNN 3.x.  However, FCK seems to choke when a Text/HTML module is fairly large and has some broken tags. 

No, I don't believe that this has anything to do with WIKI or other modules.  We experience the problem with Text/tHTML modules.

When we go to edit some old, large Text/HTML modules, we sometimes get the following error in the browser:
Line:  36
Char:  598
Error:  Invalid argument
Code:  0
URL:  http://www.mydomain.Com/Providers/HtmlEditorProviders/Fck/FCKeditor/editor/fckeditor.html?InstanceName=dnn_ctr1011_EditHTML_teContent_teContent&Toolbar=DNNDefault

Thanks, to Ven's posting, we are able to fix each module by cleaning up the tags.  However, it would be nice if the FCKEditor were a bit more rugged with dealing with broken tags.  Broken tags happen naturally as end users continually update the same Text/HTML module.

We've got the solution to fix the broken down to a relatively simple procedure, which is similar to Ven's solution:

• Copy the text out of the Text/HTML module.
• Paste it into another HTML Editor.
• Wipe out the text in the problem module, and save the changes.
• Copy the text back into the Text/HTML module and save the changes again.
• Apparently, FCK does fix up the tags using this procedure.

Here's a play-by-play in excruciating detail, in case anyone needs it.  Yes, we use FrontPage 2003 as our intermediate HTML editor.  No, we don't let FrontPage 2003 embed any gratuitous code.

Assumptions
     My URL:                                                             www.mydomain.com
     Page on which problem module exists:     Home -> Page1
     Text Module with Problem:                            Text1

FrontPage 2003
     I started a new page.
     Code View
          I selected all FrontPage code and wiped it out.
     Design View
     File -> Save As -> ..\Desktop\test1.htm

IE -> www.mydomain.com
     I logged in.    
     Home -> Page1
          Text1:
               I edited this module.
                    Design View (not “Source” view)
                         I selected all of the text.
                              <CTRL><A>
                              <CTRL><C>

FrontPage 2003 -> test1.htm
     Design View
          I pasted the text copied from the “Text1” module.
               <CTRL><V>
     File -> Save

IE -> www.mydomain.com
     Home -> Page1
          Text1:
               This module was still in edit mode.
                    Design View (not “Source” view)
                         I delete all text in this module:
                              <CTRL><A>
                              <DEL>
                         I typed a single word:
                              Test
                         I clicked “Update”
                              The save was successful.
          Text1:
               I edited this module, again.
                    Design View (not “Source” view)
                         I selected the existing text (“Test”) and deleted it.

FrontPage 2003 -> test1.htm
     Design View
          I selected all of the text and copied it:
               <CTRL><A>
               <CTRL><C>

IE -> www.mydomain.com
    Home -> Page1
          Text1:
               This module was still in edit mode.
                    Design View (not “Source” view)
                         I pasted the text from FrontPage:
                              <CTRL><V>
                         I clicked “Update”
                              The save was successful.

One point to note is that copying the text to an intermediate HTML editor is not necessary.  I just do it be safe.  In reality, you can simply copy the text to the clipboard and then copy it back to the Text/HTML module.  However, I always remember one of Dirty Harry's phrases, "Do you feel lucky, Punk?"  The problem modules are always quite large, so I save the text because I typically don't feel too lucky. 

Hopefully this will help others who suffer with this issue.

See Ya,
Van