I'm getting the following error when the user doesn't even exist in CC.
CTCT.Exceptions.CtctException: query.param.action_by.action_by_owner.forbidden:The contact can not be opted in using 'ACTION_BY_OWNER'.
I have similar users that I try to add and one will add and the other won't.
I have replied to this same question you asked in the other thread that you asked. You can find my answer here: https://community.constantcontact.com/t5/Getting-Started-with-API-s/Error-when-updating-contact/m-p/...