Server Response Error: 'Unknown Server error' for async file upload control

See more:

C#

C#4.0

When click on browse button of async file Upload Control i got the following error
Server Response Error: 'Unknown Server Error'
Do You want to see the response page?
Whats root cause ?Here is my Code

protected void fileUploadComplete(object sender, AsyncFileUploadEventArgs e)
   {
       if (fileUpload1.HasFile && IsValidPhoto(Convert.ToInt32(fileUpload1.PostedFile.ContentLength)))//Checks Valid Photo Type or not
       {
           Thread.Sleep(1000);
           string newfilenameWithExt = "";
           string ext = System.IO.Path.GetExtension(fileUpload1.FileName);
           Bitmap bmpImg = null;
           newfilenameWithExt = hdnFileNmWithoutExt.Value + ext;//hidden field value is set on Page load
           if (hdnOldFileName.Value != "")//Old Picture is removed
           {
               string oldFilePath = Server.MapPath("~/") + "/storedata/" + "studentphotos/" + hdnOldFileName.Value;
               FileInfo oldFile = new FileInfo(oldFilePath);
               if (System.IO.File.Exists(oldFilePath))
                   System.IO.File.Delete(oldFilePath);
           }

           // Resize Image Before Uploading to Storedata
           bmpImg = Resize_Image(fileUpload1.PostedFile.InputStream, 97, 120);
           bmpImg.Save(Server.MapPath("~/") + "/storedata/" + "studentphotos/" + newfilenameWithExt, ImageFormat.Jpeg);

           Session["ProfileImg"] = newfilenameWithExt;

           loadStudentProfilePicture();//method to load studentprofilepicture
           HttpResponse.RemoveOutputCacheItem("/personalDetails.aspx");
           StudentApplication objStdFileUpload = new StudentApplication();
           objStdFileUpload.StudentFormID = 0;

           if (Session["FormId"] != DBNull.Value && Session["FormId"].ToString() != "")
               objStdFileUpload.StudentFormID = Convert.ToInt32(Session["FormId"].ToString());
           objStdFileUpload.StudentProfileImage = newfilenameWithExt;

           if (objStdFileUpload.StudentFormID != 0)
               objStdFileUpload.UpdateFileNameUploadByFormID();
       }
   }