This happens when Domain 1 has DNS record correctly pointed to the right installation. Domain 2 is an old domain or domain that does not exist. When requesting Domain 1 in the browser, Domain 2 is provided. As a result, no content is available.
This can occur when the NetCommunity web.config file has an entry for the certificate domain. The certificate domain key causes the app to direct all requests to the domain contained within the key.
Continuing our scenario, this would occur if the web.config entry appeared as follows: <add key="CertificateDomain" value="Domain 2" />
To resolve, the key should either be updated to the new certificate domain, or commented out if it is no longer applicable.