Thanks for the suggestion. I have:

Deleted ~/.config/linfati.com and ~/.config/linfati (old version) config directories.
Revoked access to the app in Google account settings so they're recreated from scratch.
Reauthenticated via the "Auth" button in the application, allowing access and getting to the "OAuth finished" screen.
Closed app and started /opt/linfati.com/bin/LatitudeUpdaterDaemon from console to get debug output

Same errors:

"You haven't given <number>.apps.googleusercontent.com any permission to access your Latitude location information." (from Google account settings page)

* GoogleLatitude::onRequestReady "{"error":{"errors":[{"domain":"global","reason":"insufficientPermissio ns","message":"The user is not opted in to Google Latitude."}],"code":403,"message":"The user is not opted in to Google Latitude."}}"
...
* GoogleLatitude::onRequestReady "{"error":{"errors":[{"domain":"global","reason":"backendError","messag e":"Backend Error"}],"code":503,"message":"Backend Error"}}"

Thanks.