If your Lumia is on Windows 10 higher than 1703, you can follow the Interop procedure below.

  1. Install 8 Zip from Microsoft Store
  2. Install Interop Tools to unlock Lumia
  3. Create WhatsApp backup with App Data Manage Tool
  4. Extract messages from WhatsApp backup

If App Data Manage Tool crashes or the created backup is empty (0kb), kindly try a factory reset of the Lumia. If still not working, you can try the older procedure without App Data Manage Tool