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.