RDS Farm Clients - Dual credential entry required

EDIT - I've located this answered question which hold some similarities - for TS Farm but same symptoms - any other infor still appreciated.

https://www.experts-exchange.com/questions/26627565/Terminal-Server-Farm-Connections.html

We have an issue with a client using an RDS Farm and HP t5740Thin clients running WIndows 2009 Embedded Standard.

Their current RDS environment is:

· RDS01 and RDS02 – RDS Session Hosts running Windows 2008R2, identical virtual machines, identical apps, part of farm “RDSfarm”. Using self-signed certificate on both hosts

·  WDS01 – RDS Licensing Server and Connection Broker and Session Host configured for dedicated redirection. Using self signed cert.

· All servers are on the same subnet as each other and connecting clients


 

Symptom: When a thin client is pointed at RDSfarm the user is prompted to authenticate by one of the session hosts, they enter credentials, the screen blanks momentarily and then they are presented with the login screen again. If they re-enter credentials, they are logged onto one of the session hosts as expected.

 I have read through sections of Microsoft’s Remote Desktop Services Resource Kit pertaining to setting up server farms and how the connection broker functions. What I can conclude is that the client (thin client or otherwise) is connecting to one of the RDS session hosts, authenticated, is then being pointed at the connection broker, the broker decides which session host to connect them to, and then they are presented with the login screen on the “brokered” session host.

Initially, the farm was setup using round-robin DNS load balancing.

I have also tried a setup with WDS01 configured as a dedicated redirector but the symptoms remain the same.
 

My research online has revealed other people with similar issues while using an RDS farm -

 http://social.technet.microsoft.com/Forums/en/winserverTS/thread/eaec24a5-e99b-4add-9a76-a3a87b251a31

http://social.technet.microsoft.com/Forums/en-US/windowsserver2008r2rds/thread/6acd6014-2de4-4648-8d23-671d1544e92e

 

At present I am pondering if this is an issue caused by one of:

-Thin clients not being domain joined?
-CredSSP settings on Thin Client?
-Requirement for Trusted certifcate to allow session brokering to work as expected?

We know that the thin clients haveMicrosoft Remote Desktop Connection 6.1.7600.16385 to support Remote Desktop Protocol v7.0.

 Please let me know if I can provide further detail for troubleshooting. Any help appreciated.