Federation with WAAD as IP-STS; replies to wrong URL!

Hi,

 This is driving me insane; current setup:

  IP-STS: A WAAD/O365 tentant
  R-STS: Thinktecture IdentityServer v2 hosted in an Azure Cloud Service.
  RP: An MVC app @ localhost at the moment.

  I've configured the WAAD as an Identity Provider in Thinktecture IdSrv (according to Dominick's blog post: http://leastprivilege.com/2013/06/24/adding-windows-azure-ad-ga-as-an-identity-provider-in-identityserver/ )

  The RP is configured as an RP in IdSrv.

  Now, when I fire up the RP, I get forwarded to the HRD page of IdSrv. All good. I choose the WAAD tenant, and I get redirected to the WAAD login page, where I identify myself with my credentials. All good - Except:

The reply isn't sent to the Reply URL I've configured in the WAAD App Integration page for the IdSrv: https://myidsrv/issue/hrd/ , but to the root of the web site, https://myidsrv/ !

I've looked through all config settings and inspected the WS-Fed flow with Fiddler, but I can't make out why it doesn't reply to the right URL :-/

Anyone?