I was receiving the Sys.Webforms.PageRequestManagerServerErrorException error with an ASP.NET AJAX project I was working on. I searched the Forums and other sources and found some common recommendations, but none worked. Here's a summary of the cause and solution:
1) The web.config had the customErrors tag mode="RemoteOnly" and a custom Error page. This caused the exception to happen with server deployments, but not show itself with local development environments.
2) The custom Error page causes the Response to AJAX Request to be altered and therefore causing the PageRequestManagerServerErrorException
3) Setting the AllowCustomErrorsRedirect="false" on the ScriptManager prevents the custom Error page from being used with AJAX Requests - http://msdn.microsoft.com/en-us/library/system.web.ui.scriptmanager.allowcustomerrorsredirect.aspx.
4) Define an OnAsyncPostBackError event handler on the ScriptManager to handle exceptions and set the OnAsyncPostBackErrorMessage to a user friendly error message the can be displayed to the user - http://msdn.microsoft.com/en-us/library/system.web.ui
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