RD Gateway crashes every time a user tries to connect through

Hey guys!

We installed RDS on our Windows Server 2008 R2 SP1 with RemoteApp and RD Gateway.
Everything seems to work - we tried user-connection with rdweb and the deployed Remote-App-rdp-links.

But then we tried to log in with mstsc. I changed the "connect from aynwhere"-settings in the advanced-tab and forced the application to use the specified RD-Gateway.

As soon as we typed in the credentials and click "connect" - the RD Gateway-Server crashes with the error-code: 3221225477.
On the support-site of microsoft I discovered the article "The Remote Desktop Gateway service crashes under a heavy workload in Windows Server 2008 R2" with a hotfix.
Even though we don't have this heavy workload (well, we have the second a user get's successfully logged in, but I guess that's normal...), I installed the hotfix, but the problem remains...

Furthermore the event-log-entry said the service will restart on it's own, which it doesn't...

Thanks for any help or ideas!

Cheers,
Richard