Hi @emlee, I got some clarification on what happened with these contacts. Your account has had Marketing Pro since 2016, which means that you should have been grandfathered into the "Trigger a Webhook" action on November 1, 2018. Many accounts, however, were not grandfathered into this action until later in the day, when the team realized that eligible portals did not have access to all of the actions they should have. The errors that you identified were related to the brief lack of access to the webhook action.
Since your account has been grandfathered into the action, we can therefore be certain that you will not run into these errors moving forward.
To fix the affected contacts, you should be able to enable manual re-enrollment for this workflow and manually re-enroll the affected contacts into the workflow to ensure that the webhook step is completed as expected. Let me know if you have questions!