Connection to remote desktop/apps failing via RDWeb

Hi,

2012 R2.

I have connection broker, gateway and web access on same box.  A collection has been create with access to two session hosts.  The rdp connection is presented in RDWeb.  When clicking on this it attempts to connect but immediately fails.  If I manually create an RDP session to one of the session hosts in the collection and specify the gateway server it works correctly.

Error wise all I see on the CB is:

TerminalServices-SessionBroker-Client

Remote Desktop Connection Broker Client received request for redirection. 
User : xxxx
RDP Client Version : 5

Remote Desktop Connection Broker Client failed while getting redirection packet from Connection Broker.
User : xxxx
Error: Element not found.

Remote Desktop Connection Broker Client failed to redirect the user xxxx
Error: NULL

under TerminalServer-SessionBroker

RD Connection Broker failed to process the connection request for user xxxx
Error: Element not found. 

So this all looks like a connection broker issue but I cannot see any more clues as to what is causing it...

On the session host the following is logged:

Remote Desktop Connection Broker Client failed while getting redirection packet from Connection Broker.
User : xxxx
Error: Remote Desktop Connection Broker is not ready for RPC communication.

there is a firewall between the CB and the session hosts so perhaps a required rule is missing?

Any ideas?

Thanks