I had an instance recently where the FCKeditor would not display in any browser. Instead, I was greeted with nothing in non-IE browsers, and in IE I was greeted with the unwanted "Internet Explorer cannot display this webpage" page.
I could not figure out why this was happening... Upon getting advice from the DNN forums, I tried checking the file permissions, reinstalling the provider, and more. Nothing seemed to work.
Then, the lead for the project team suggested that I look at the IIS configuration. BINGO! That was it. For this particular site, I had the HTML pages being rendered by the ASPX process. (I know, I know... but I had a REALLY good reason to do so.)
Removing the HTML pages from the ASPX process did the trick!