portal accessing internal https sites

I've created several portal access rules accessing various internal websites. These work without an issue but are only http sites.

 

When I create a rule where the internal site is using https it does not load. When I click on the link it times out with the message pages can not be displayed. I know it's https issue, if I change the rule to use http I get a internal server message of forbidden which is what I get if I access the page on the internal domain.

 

Does anyone know why this would be the case or something I can try?

 

Thanks