Some context would be nice; what device are you getting this error on, and what were you doing when it occurred (or doing to provoke it!)? In the meantime, try putting the error into a Google search, or a search on this Community, which will turn up:-
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.
