IPS Causes Exchange OWA to stop working

Hi,

I've used Microsoft TMG in the background for reverse proxy and fortigate 200-E ahead of it for publish from Internet.

When I enable IPS for the publish rule on Forti, I see this on browser

The page you are trying to view can not be shown because the authenticity of the received data could not be verified.

Unfortunately the log&reports does not show any logs (not working at all although I've checked all logging options and I don't really know what to do more to enable logs)

Any help appreciated.