Why is it taking 3 login attempts to access my external server? SSL CA signed cert?

Hi all,

I am currently running ArcGIS Server 10.1 SP1 with the web adaptor. Publishing and access works fine on the internal network. However it takes 3 login attempts to access the external reverse proxy address. It prompts me for a user name and password each time I access a folder via the services directory. Is this something to do with not enabling SSL using a CA signed certificate?

Regards

Alex