The OAuth handshake for Live mode sends your site's URL (home_url()) to wpdirectdebit.com/, which checks it against a subscriber registry before allowing the flow to proceed — this is how the license/subscription is tied to a specific site. Sandbox connections skip this check entirely, which is why sandbox testing "just works" on any domain, including a fresh staging clone. If you need Live credentials on a staging copy, don't try to OAuth from the staging URL — copy the already-encrypted wpdd_access_token option (and matching WPDIRECTDEBIT_SALT) across from the source site instead, or just work in Sandbox on staging and switch to Live only on the real domain.