HttpException: File Does not exist

Hello Friends,
I am using Asp.net 3.5. In my application I am handling errors at application level in Global.asax.
I am using 3rd party control FpSpread in my application. In all the pages where I've that control, it's throwing
"File does not exist" error when first time postback occurs in IE (on every postback in FF).
From the log I got following Details:
04/01/2010 11:32:18 : The error description is as follows :
URL- http://localhost:1348/Forms/Company/~/App_Themes/Blue/style.css
Source- System.Web
Type- System.Web.HttpException
Message- File does not exist.
Stack Trace-    at System.Web.StaticFileHandler.GetFileInfo(String virtualPathWithPathInfo, String physicalPath, HttpResponse response)
   at System.Web.StaticFileHandler.ProcessRequestInternal(HttpContext context)
   at System.Web.DefaultHttpHandler.BeginProcessRequest(HttpContext context, AsyncCallback cal ...