Fortiweb - Server Policy for Exchange OWA - "The service of this port is in use"

I'm following the cookbook.fortinet.com guide "publish outlook web app owa using fortiweb" and have gotten to seemingly the end of the OWA piece on #9 "Server policy". I fill in all the fields as specified in the document and I end up with this:

"The service of this port is in use"

My virtual server is using interface/port1's IP; this port is *not* used for management and everything but PING is turned off on that interface in regard to management.

Each time I get this error, the Edit Policy form resets my selection specifically for Server pool.

The Server Pool selection I've defined is as follows:

Protocol: HTTP; Type: Reverse Proxy; Single Server; IP = Exchange2010CAS NLB cluster IP; port 443; HTTP2 disabled

It may just be a coincidence that it resets this field, I'm not sure.

Has anyone run into this? Is there a better guide to follow for OWA pass through setup? etc.