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The iPhone app keeps telling me I'm trying to scan an authentication token barcode that isn't valid, what to do ?

Apparently the iPhone app won't accept a barcode containing space characters in the description, removing space characters in the description should fix the problem.

Can I use Google Authenticator for WordPress with the Android/iPhone apps for WordPress?

Yes, you can enable the App password feature to make that possible, but notice that the XMLRPC interface isn't protected by two-factor authentication, only a long password.

I want to update the secret, should I just scan the new QR code after creating a new secret?

No, you'll have to delete the existing account from the Google Authenticator app on your smartphone before you scan the new QR code, that is unless you change the description as well.

I am unable to log in using this plugin, what's wrong ?

The Google Authenticator verification codes are time based, so it's crucial that the clock in your phone is accurate and in sync with the clock on the server where your WordPress installation is hosted.

If you have an Android phone, you can use an app like ClockSync to set your clock in case your Cell provider doesn't provide accurate time information

Another option is to enable "relaxed mode" in the settings for the plugin, this will enable more valid codes by allowing up to a 4 min. timedrift in each direction.