Starting an RemoteApp through RD Web Access and RD Gateway fails with “The logon attempt failed”

In Windows Server 2008 R2, it`s possible to use RD Web Access and RD Gateway for starting RemoteApp applications, using Single Sign-On (SSO).
The Remote Desktop Services Team has a great article here on how to configure this setup.

While troubleshooting a problem with the SSO-feature lately, we got an error message saying “The logon attempt failed”.

The solution turned out to be rather simple. While the default logon page for RD Web Access indicates “Domain/user name” for the user name field, using only the user name works fine:

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This works fine as long as your not using RD Gateway and SSO, then the error message mentioned will occur when trying to start a RemoteApp.
When RD Gateway and SSO are being used, you must supply the domain as part of the user name (domain user name), else the SSO-feature will break.

I actually contacted Microsoft to clarify if it is possible to hardcode the domain name some way, and got the following answer:

I have taken time to discuss with our develop team. Please understand that there is no easy way to finish the task, because in RemoteApp mode RDP client ignores user name and domain name. We need to put a domain hint on the RD Web page. It is related to coding and design. If you want to request a feature or code change, you also can contact our Advisory Service by using the contact info published at <http://support.microsoft.com/gp/advisoryservice>.

Hopefully this will be possible in future Service Packs or OS releases.