The Act-On error 900904 indicates that the access token being used for the request has expired. If the bulk download is a very long running job it needs to be noted that the Act-On access_token expires 1 hour after the time it was granted, but there can be other reasons for the access token to expire during the bulk download as well.
The main possible causes are that:
1) Users are revoking access tokens from each other while the bulk job is in progress, which can happen if additional API calls are made against the same Act-On credentials while the bulk job is running. Alternatively, if an Event Poller is configured on the same element instance then every time the poller runs it makes a call to GET /{objects} and this can invalidate the token on the bulk job already in progress as well.
2) If we are trying to refresh the Access token but the refresh token has already been revoked by a new OAuth connection to Act-On, this 900904 access failure could also occur.
3) If the token used for the bulk request was subsequently revoked by a new user, but it hasn't hit the refresh interval yet, we send a revoked access token during the bulk request.
