This should be
for member in committee.members.exclude(committeemembership__until__lt=timezone.now().date()):
From the error log:
Internal Server Error: /mailinglists/ FieldError at /mailinglists/ Cannot resolve keyword 'commiteemembership' into field. Choices are: address_city, address_postal_code, address_street, address_street2, bank_account, birthday, committee, committeemembership, direct_debit_authorized, display_name_preference, emergency_contact, emergency_contact_phone_number, event_permissions, id, initials, language, mailinglist, mentorship, nickname, order, phone_number, photo, profile_description, programme, receive_newsletter, receive_optin, registration, show_birthday, starting_year, student_number, user, user_id, website
mentioned in issue #225 (closed)