This error occurs when the Mobenzi mobile app is deleted (uninstalled) from a handset and then re-installed. For security reasons, a token is automatically generated and assigned to the handset when the app is installed to prevent unauthorised third-parties from emulating the handset and potentially accessing or submitting data.
To resolve, locate the handset from the Handsets tab and click 'View handset' from the menu. Click "Clear" next to the token - a new token will be generated when the handset next connects to the server. Ask the fieldworker to check for form updates once you have cleared the token.