ASP.NET Error: Sys.WebForms.PageRequestManagerServerErrorException?

I have a 3.5 VS dot net web C# application that is returning the  "Error: Sys.WebForms.PageRequestManagerServerErrorException? Exception has been thrown by the target of an anovation"  and want to know how to resolve this error an/or where to start at in order to determine what's causing this error? any help would be great.
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Umar Topia.Net Full Stack DeveloperCommented:
What are you exactly doing when this error is thrown?
looks you are using AJAX .. just check if that part is configured properly. The error handling is a bit different if AJAX is being used on web pages.

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