Our team member is failing to login - while trying to use “Forgot your password?” option the service returns “This account uses an external authentication source. Impossible to change the password.” What is the practical guidance to get forward with this account?

Hi Jari,

your team member probably signed up with his / her Google account (external authentication source). Since in this case Google manages the permissions (including passwords), the error message is shown.
On the login screen there is the option to sign in with Google. Your team member should be able to sign in selecting this option.