We have an ADFS 2.0 instance set up. We use it for 3rd party web app single sign-on. Everything works beautifully with the existing app, App1 with SAML 2.0, including IWA pass-through when users are redirected to our ADFS server.
I just configured a second Relying Party Trust for another application, App2, using SAML 2.0. We're using all of the default ADFS settings, including endpoints. On this application's SAML configuration page, I've told it that our SAML endpoint is "https://adfs.mydomain.com/adfs/ls/". Everything works fine, except that users are prompted for credentials; ADFS is not using IWA for these logins.
I'm testing from a local domain-joined workstation using IE9. Internal DNS points to our local domain-joined ADFS server, external DNS points to our DMZ ADFS proxy. "*.mydomain.com" is in the Trusted Sites zone in IE by GPO and is applied. IWA is enabled in IE. IE clears its cache/cookies/history every time I close it. Both applications are using SP initiated sign-on and both send their assertions to the same endpoint URL on our end.
If I attempt to log in to App2 on a clean session, I'm presented with an ADFS login page. If I enter credentials, I'm logged into the app and can proceed.
If I attempt to log in to App1 on a clean session, I'm immediately redirected to ADFS, IWA is passed through, and I'm logged into the app and can proceed.
If I attempt to log in to App2 in the same session after logging in to App1, I'm redirected to ADFS, the existing ADFS login session initiated by App1 is used, and then I'm immediately redirected to the App2's assertion consumer service URL and given an error page.
My best guess is that I'm missing something in the RPT configuration. All that the SP has given me is their consumer service endpoint URL using a POST binding. I had to guess at the claim rules. The SP doesn't use encryption.
App2 Customer service has been... difficult. Their tech support is overseas, so I only get responses once a day around midnight local time. Most of their responses are standard "copy and paste from the KB". They prefer IdP initiated sign-on, but say they support SP initiated - which seems to be true, since SSO does work on a clean session after logging in on the ADFS login page.
Does anybody know what I'm missing?