App Data Manage Tool is a free tool provided on XDA that let's you perform an unencrypted backup of your WhatsApp. As a pre-requisite, make sure you performed the previous step Interop Unlock otherwise the app will not install.

  1. download App Data Manage Tool on your Lumia by clicking here
  2. touch the downloaded file then tap/click
    1. all the files inside Dependencies folder
    2. AppDataManageTool_2.2.0.0_ARM.appx (the app itself)
  3. force-close WhatsApp if currently running
  4. open App Data Manage Tool and perform a backup of WhatsApp

If your backup is 0KB in size, double check you have enough free space on the phone and if it still doesn't work try a factory-reset, this seems to fix most issues. Please note, you'll need to backup WhatsApp on OneDrive before, in order to restore after the reset.