Hi,
This is really strange. The relativeSite Url is concatenated to the server url specified in the datasource connexion string (eg: server=http://SharePointServerUrl/).
Are you using a Shared or embedded datasource? What is the value of the connexion string? Would you possibly have something like
"server=" without any serve url after the equal sign?
You may want prefer sending an email to our support address (see the exact address in the product documentation) if you would like to provide us with some screen shot and/or your report server file (.rdl) as well as your data source file (.rds) in case you are using a shared datasource.
Regards,