I found that everytime I added the irule above I got page can't be displayed after successfully activating and processing the access policy on the virtual server.
After logging a support call and going thru my config several times with support it just wouldn't work.
I had the idea to write an irule to only disable the access policy - not enable it because it is enabled by default.
If the path doesn't match the url disable the access policy as per below
when HTTP_REQUEST {
if { not ([string tolower [HTTP::path]] equals "/owa/calendar/user@mydomain/calendar/calendar.ics") } {
ACCESS::disable
log local0. "APM Disabled"
}
}
This works!