When you installed Aggregate, you specified a FQDN (fully qualified domain name). You must use that address (and port) when connecting to Aggregate. The form download/upload (but not the form listing) relies on the FQDN so you'll get errors like you describe.
The easiest way to change the FQDN is to re-run the Aggregate installer and put in the new FQDN. Another option is to manually edit security.server.hostname and security.server.port in security.properties (it's in webapps/ODKAggregate/WEB-INF/lib/ODKAggregate-settings.jar). How to change the odkaggregate ip address - #5 by Saad has a bit more on this.