Thanks again guys for your help. Chris, I meant that it worked when running on another machine. Sebastian, I have tried copying the fckprovider section from the DNN 4.9.0 upgrade release config and also copying the FCK folder from the HtmlEditorProviders in the upgrade release as well but it still does not work on my VPS.

However, I have been able to go back to FCK 1.0.9 by coping the the FCK folder and DotNetNuke.FckHtmlEditorProvider.dll from a backup of DNN 4.8.4 and FCK 1.0.9 is now working fine with DNN 4.9.0. Definitely a weird problem and I will ty to get FCK 2.0.2.1 working again later but the client need it fixed so I jsut had to get it working for now.

Chris Lively wrote   We are experiencing the exact same problem.  A couple vista machines with FF 3 and IE 7 installed only see white space instead of the editor.  Also, they are receiving the same error messages. Other machines have absolutely no issues. Right now the only thing I can think of is Virus scanning / spyware software. What were you running?

I am running Vista Ultimate, AVG Anti-Virus, Comodo Firewall, and Windows Defender on my PC. However I don't think any of these were causing the problem as I could get FCKEditor working on my PC (when using localhost), but when I copied the DNN site from my PC over to my VPS webserver FCK Editor would not work on my PC or VPS which makes me think it was a configuration issue on my VPS. As already mentioned I have rolled back to FCK 1.0.9 for now which works fine on my VPS and PC so you may want to try that. It is a weird issue and I'm surprised that not many people are having problems with this, however if you find a fix please post it here.

I'm narrowing down the problem, but it's getting weird. 

I have multiple portal aliases to my sites.  The editor will work with some of the aliases, but not others.  Even though the aliases point to the exact same site.

I found that the ASP.NET_SessionID cookie is not being assigned by the sites where the editor is failing.  I'm not sure why that cookie isn't being created, but it appears to be the real problem.