An unexpected error occured while connecting to the report server. Verify that the report server is available and configured for Sharepoint integrated mode. --> Server was unable to process request. ---> Client found response content type of 'text/html; charset=utf-8', but expected 'text/xml'.
Check the Report Server websites has started in the IIS.
Restart the SP timer services and SSRS report services.
Check the SQL log files and account could access the SSRS reports.