OAuth get token can't open page

We have same issue here. The machine that is running SoapUI NG and the authorization server are on the same intranet. When clicking the get token button on SoapUI NG “Get Access Token from the authorization server” popup, the browser opens up with a "Unfortunately, we can't open this page!" message. The authorization server is accessible within the intranet using browsers like Chrome and Safari, but it is not accessible to connections from outside of the intranet.

We tried with Google OAuth using Implicit Grant flow where the authorization server is accessible from any network connections and everything worked fine. In our own test we use Authorization Code Grant flow. It is our speculation that the authorization server thinks the traffic is coming from an untrusted source (SoapUI NG) and thus blocked it.

The questions are:
1. Is this a bug in SoapUI NG? Note that both SoapUI NG machine and authorization server are on the same intranet, so they are able to connect with each other.
2. If it is not a bug, what might have caused this issue and? Is there a way to get around this? It doesn’t make much sense that to test a 3-legged OAuth in development requires the authorization server to be accessible from any network connections.

Thanks,

Mojo