Remote Desktop App - Error 0x207 or 0x607

Hi All,

I manage a Windows 2012 R2 Network, including Connection Broker, Session Hosts and Remote Apps virtual servers. 

I've used the Remote Desktop app on my own Android phone, with no issues at all - Connects to the published gateway URL and I can login and access network.

We are looking to move the business onto Nokia handsets with MS operating system (currently on Blackberry), but not able to connect via the Remote Desktop app on the Nokia Lumia handset.

Setup, as per Android config and it brings up my Remote Apps and a Remote Desktop screen, so I know it can connect. 

When I then select the Remote Desktop, it shows following process:

1. Initiating Remote Connection
2. Securing Remote Connection
3. Certificate can't be verified - I choose Connect
   1. It is a digitally signed cert (DigiCert SHA2), but the PC name is different to the physical Connection Broker name (inherited domain name which was an old GOV project - can't publish it publicly)
4. Initiating
5. Establishing
6. Securing
7. Connection Error Most have been 0x207 but now getting 0x607

A bit baffled, as will work on Android (and iPad at home using the app). Assume MS use a different way to authenticate the connection? 

Hope someone can help.

Thanks,

Darren.