public static ReportDocument rep = new ReportDocument();
   protected void Page_UnLoad(object sender, EventArgs e)
   {
       this.CrystalReportViewer1.Dispose();
       this.CrystalReportViewer1 = null;
       rep.Close();
       rep.Dispose();
       GC.Collect();
   }

2. crystal report assemblies installed on your machine do not reflect the assemblies you have on your web.config

3. crystal version installed on the server is different with the one you want to invoke from your site

4. application pool where your reports run is not set to LOCAL SERVICE

5. read and write permission is not enabled on your root folder

6. crystalreportviewer folder is not copied to your operating folder. 

7. lack of permission in following  directory

The fix: When Crystal Reports opens a file, it uses the Windows temporary folder (typically C:\Windows\Temp\) as a scratch-pad. You need to give Crystal Reports explicit permission to read and write to this folder.

How-to: Under XP, ASP.NET runs CR under ASPNET; with most of the Windows Server flavors, CR runs as NETWORK SERVICE. Make sure that this identity has permission to read and write to the Windows temporary folder.
1. Ensure the RPT file name is correct. Filename must be a fully qualified valid path name (such as C:\MyApp\MyReport.rpt).
2. The ASPNET account has permissions to open the file.  Make sure the folder containing the file and the file itself has the same permissions as your application folder.


But still i don't found any solution
next i tried to discover my iis different settings. and i found an interesting option.
* select your application pool for your apllication or website
* Click on Recycling (Right side panel)
* Reduce the value of Regular time interval 1740 to 20 min or set your desire value you want
* this  will release any resource hold by iis.

And This has help me to overcome this error