A recent change to CommCare HQ will prevent Data Forwarding to localhost. On a multi-tenant environment like www.commcarehq.org there is no legitimate purpose for forwarding to a port on localhost or a private IP address, and that was the basis for the change. However, we will be evaluating whether this affects developers who have a legitimate use for pointing a Data Forwarder to localhost on a private single tenant instance.
If this applies to you, please be advised that until further notice, updating CommCare HQ on your instance will cause Data Forwarding to localhost to result in an error.