I have just fixed the same "File not Found" exception in my own code (although nothing to do with CuteEditor!)

 

To track down _which_ file is not being found, within your Application_Error catch logic, examine the Url which is throwing the error:

 

    string url = HttpContext.Current.Request.Url.ToString();

 

Now carefully examine the generated page code for this Url .... either the Url itself is in error or a "<link ...>", "<script..."> or any other link to a resource could be awry - In my case it was a badly pathed "<script type="text/javascript...> tag.

 

I corrected the path, and the exception disappeared!

 

Hope this helps