Document upload issue

[Victor]

Hi Telerik!

There's a nonrecurring issue with uploading documents.
In some cases (10-15%) an attempt to upload a document fails (it doesn't depend on a specific document, I can successfully upload the document which failed a second ago, or I can't upload the document which I successfully uploaded a second ago). I see the red light
image and the debugger shows that the following exception occurred:

Invalid JSON: <SPAN>
<H1>Server Error in '/' Application.
<HR color=silver SIZE=1 width="100%">
</H1>
<H2><I>Object reference not set to an instance of an object.</I> </H2></SPAN><FONT face="Arial, Helvetica, Geneva, SunSans-Regular, sans-serif "><B>Description: </B>An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. <BR><BR><B>Exception Details: </B>System.NullReferenceException: Object reference not set to an instance of an object.<BR><BR><B>Source Error:</B> <BR><BR>
<TABLE width="100%" bgColor=#ffffcc>
<TBODY>
<TR>
<TD><CODE>An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.</CODE> </TD></TR></TBODY></TABLE><BR><B>Stack Trace:</B> <BR><BR>
<TABLE width="100%" bgColor=#ffffcc>
<TBODY>
<TR>
<TD><CODE><PRE>
[NullReferenceException: Object reference not set to an instance of an object.]
   System.Web.Util.StringUtil.memcpyimpl(Byte* src, Byte* dest, Int32 len) +15
   System.Web.Util.StringUtil.UnsafeStringCopy(String src, Int32 srcIndex, Char[] dest, Int32 destIndex, Int32 len) +80
   System.Web.HttpWriter.Write(String s) +66
   System.Web.HttpResponse.Write(String s) +15
   Telerik.Web.UI.AsyncUpload.ResponseWriter.WriteToResponse(String response) +54
   Telerik.Web.UI.AsyncUploadHandler.ProcessUploadedFile() +171
   Telerik.Web.UI.AsyncUploadHandler.ProcessRequest(HttpContext context) +203
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&amp; completedSynchronously) +75
</PRE></CODE></TD></TR></TBODY></TABLE><BR>
<HR color=silver SIZE=1 width="100%">
<B>Version Information:</B>&nbsp;Microsoft .NET Framework Version:2.0.50727.5466; ASP.NET Version:2.0.50727.5456 </FONT><!--
[NullReferenceException]: Object reference not set to an instance of an object.
   at System.Web.Util.StringUtil.memcpyimpl(Byte* src, Byte* dest, Int32 len)
   at System.Web.Util.StringUtil.UnsafeStringCopy(String src, Int32 srcIndex, Char[] dest, Int32 destIndex, Int32 len)
   at System.Web.HttpWriter.Write(String s)
   at System.Web.HttpResponse.Write(String s)
   at Telerik.Web.UI.AsyncUpload.ResponseWriter.WriteToResponse(String response)
   at Telerik.Web.UI.AsyncUploadHandler.ProcessUploadedFile()
   at Telerik.Web.UI.AsyncUploadHandler.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
--><!--
This error page might contain sensitive information because ASP.NET is configured to show verbose error messages using &lt;customErrors mode="Off"/&gt;. Consider using &lt;customErrors mode="On"/&gt; or &lt;customErrors mode="RemoteOnly"/&gt; in production environments.-->

I use a custom http handler, yet I'm pretty sure that it's not the reason of the exception due to the call stack.
Besides, if it were it would throw this exception at 100% rate.

Can you help?

Thanks,
Victor