You need to unlock your Windows Phone through Interop since there's no other way to access the unencrypted WhatsApp backup:

  1. enable Developer mode from your phone's settings (Settings -> For developers)
  2. download on the Lumia Interop Tools from this link (not from MS Store) and touch the downloaded ZIP file to open it
  3. tap/click the following files
    1. InteropTools_x86_x64_arm_LegacyRelease.cer (certificate)
    2. all the files into the Dependencies/ARM folder
    3. InteropTools_2.0.95.0.appxbundle (the app itself)
  4. reboot the phone then start Interop Tools and choose "Interop unlock" 
  5. enable NDTKSvc and reboot
  6. unlock the phone with "Interop/Cap Unlock" and reboot - on some phone the "New Capability Engine Unlock" needs to be switched too