Essentials Manager won't connect through ESRI proxy

We are setting up access to our AGS token (AD users/roles) secured services and would like to use the ability of the ESRI resource-proxy to specify the AD account to connect to the map service as.

 

This is working fine when connecting directly to the proxy in a browser but when we try to connect to the same service through Essentials Manager to add the secured service to a site, we always get a 'connection failed' error.  According to the IIS logs and the logging I added to the proxy.ashx file, the request is being completed successfully, but for some reason Essentials does not recognise it.

 

Has anyone successfully set up services this way?  Does anyone else have the same problem?

 

Any assistance would be appreciated.

 

Peter.