Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred

An ASP.NET button click event was generating this error (in client-side browser debug window):

Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred

I could not find anything server-side in the application's log file, or in IIS logs. However, in the Windows Application Log in Event Viewer, I found the exact error message (including stack trace).

Thanks to Neville for the suggestion.