This error occurs, because the UrlRoot element in the RSReportServer.config is not changed automaticly, when you change your computername or website hostheader.
Open the RSReportServer.config in the folder: C:\Program Files\Microsoft SQL Server\MSSQL.2\Reporting Services\ReportServer
Change the UrlRoot element to the correct name
Open RSWebApplication.config file and, if necessary, modify the ReportServerUrl setting to reflect the new server name. Note that this setting is not used in every installation. If it is empty, do nothing.
For example in: C:\Program Files\Microsoft SQL Server\MSSQL.#\Reporting Services\ReportServer open
For example in: C:\Program Files\Microsoft SQL Server\MSSQL.#\Reporting Services\ReportManager open
Note: It’s very important that only one of the two values is filled, else an error will occur.
Run iisreset /noforce and you’re now using multiple instance of Reporting Services on the same machine.