Access policy evaluation is already in progress for your current session.

The already in progress error page, for the most part, is triggered when you attempt to access a non-APM URI:

1. During the access policy evaluation, and
2. With a URI that doesn't match the original URI - stored in the session.server.landinguri session variable.

Not sure if this will help you, but the following should ensure that a non-APM/non-original URI does not trigger the in progress error:

when HTTP_REQUEST {
    if { ( [HTTP::cookie exists MRHSession] ) and not ( [ACCESS::session exists -state_allow [HTTP::cookie value MRHSession]] ) } {
        if { ( [HTTP::uri] ne [ACCESS::session data get session.server.landinguri] ) and not ( [ACCESS::session data get session.server.landinguri] eq "" ) } {
            HTTP::redirect [ACCESS::session data get session.server.landinguri]
        }       
    }
}

The idea is this. If an HTTP request comes in that:

1. Has the MRHSession cookie
2. Has an access policy that is not completed, and
3. The request URI does not match the landinguri variable

redirect back to the landinguri URI.