UpdatePanel "Error On Page" When Another IE Browser Window Is Opened

I am trying to us a simple UpdatePanel (UpdatePanel with a simple button and a label to return time of day).  The UpdatePanel runs fine until I open a second IE browser window.  When I return to the window with the UpdatePanel and click the button, I get an "Error on page" message and the UpdatePanel stops working.  The message is as follows: "Message: Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed.  ....      URI: http://xxxx.xxxx.com/xxx/ScriptResource.axd?d=VW0in8phYIs...."  

This doesn't happen when I test the code in localhost.  I've tried updating the Web.config to include a MachineKey and I've added ScriptResource.axd (<add name="ScriptResource" preCondition="integratedMode" verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />) to the system.webServer, but no change, the UpdatePanel runs fine until I open another IE browser window.

This is running in a shared environment, on IIS 7 servers, that uses load balancing on our intranet. 

Is this a server issue or UpadaePanel issue?  Any solutions?