iRule How to log Access Policy error messages

We use the BIG-IP (version 11) APM module to provide single sign on for a special web portal. Some of our users report issues with it, especially the error message "Access policy evaluation is already in progress for your current session" while they browsing through the portal. At the moment we do not know why this happens (it seems to be sporadically) and we will analyze it later, but first we want to know, how many users are affected (we can't ask them).

This is why we want to develop an iRule which produce a logging message when the access policy error

Access policy evaluation is already in progress for your current session

Access not found page reject message

How we can realize it? Or is there something other way to log this error message, so we can see, how many users are affected?

We already tried something like this, but it doesn't work unfortunately:

when HTTP_RESPONSE  {

      if {[HTTP::header Content-Length] > 0}{

          Default amount of request payload to collect (in bytes)
         set collect_length 3600

          Trigger collection of the request payload
         HTTP::collect $collect_length
      }
}

when HTTP_RESPONSE_DATA {

    if { [HTTP::payload] contains "Access policy evaluation is already in progress for your current session" }{

         set username (if possible), ip-adress, useragent
         set user [ACCESS::session data get "session.logon.last.username"]
        set ipadr  [IP::addr [IP::client_addr] 
        set userAgent [HTTP::header "User-Agent"]

         output
        log local0.info "DEBUG: Access policy evaluation is already in progress -> $ipadr $userAgent" 
    }
}