Sys.WebForms.PageRequestManagerServerErrorException:

Problem: - Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 500

Solution: -

In the @Page tag set 

ValidateRequest="false"

When does it occur?

This normally occurs when there is 

1) a multiline textbox in the page

2) an update panel in the page inside which the multiline textbox resides

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