Can't restore online site backup data in an offline server(LAMP)

Hi,

     I have an online server and I am running Moodle (2.7) in that.I took a backup of the entire site and I want to restore that backup in my offline server(LAMP) and it has moodle(2.8).Whenever I restore my backup I get an error "The selected file is not a valid moodle backup file and cant be restored".This error occurs even if restoration is 100% done. Is there any way to create an offline server,If so then what should be the approach?

Note: My offline server is in Ubuntu and I am running it in VirtualBox.

Thanks

KJ