The below error can be appeared while accessing Web Service URL property in SSRS configuration manager and the main reason behind is a broken\invalid <UrlString> tag.
To confirm this, go to location mentioned below and open rsreportserver.XML file in notepad. \Program Files\Microsoft SQL Server\MSRS10.SQL2008_PROD\Reporting Services\ReportServer
Correct the <UrlString> </URLString> accordingly, save the file, restart services and it should work now.
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