SSRS Error : Query execution failed for dataset 'dataSet'. (rsErrorExecutingCommand) Semantic query execution failed. Invalid object name 'RPT.*********'. (rsSemanticQueryEngineError)

I am new to SSRS and I am trying to migrate reports from 2008 to 2012. As I have so many reports to migrate, I simply got the back up of ReportServer, ReportServerTempDB, and Encryption Key and restored them to test environment. I made necessary configuration from RS configuration tool. I am able to see the reports now when I browse //hostname/reports. But when I open any particular report I am getting some error.

· An error has occurred during report processing.
(rsProcessingAborted)

• Query execution       failed for dataset 'dataSet'.
        (rsErrorExecutingCommand
    
  • Semantic query        execution failed. Invalid object name
           'RPT. ******'. (rsSemanticQueryEngineError)

****** - I am assuming this is a custom data class.

Does anyone have insight on this? or any better way that I can migrate the reports to new server with less efforts.

I don’t have the reports solution file to deploy the reports, so I have followed backup and restore process.