iRule to check for a post body

Try this version with some additional logging:

when HTTP_REQUEST {
    Send the data using local7 facility <190>
    if { not ([HTTP::cookie exists SITESERVER] ) } {
        set F2C "[string range [AES::key 128] 15 end][string range [AES::key 128] 15 end]"
        HTTP::cookie insert name "F2C" value $F2C
    }
    set cookie [HTTP::header cookie]
    HSL::send [HSL::open -proto UDP -pool syslog_server_pool_1] "<190> [IP::client_addr] \[[clock format [clock seconds] -format "%d/%b/%Y:%H:%M:%S %z"]\] --[IP::local_addr]--[URI::path [HTTP::uri]][URI::basename [HTTP::uri]]--QS=[HTTP::query]--C=[HTTP::header cookie]--UA=[HTTP::header User-Agent]--CT=[HTTP::header "Content-type"]"
    if { ( [HTTP::uri] contains "landingServlet" ) and ( [HTTP::method] equals "POST" ) } {
        log local0. "Start collecting: [HTTP::header Content-Length]"
        HTTP::collect [HTTP::header Content-Length]
    }
}
when HTTP_RESPONSE {
    if { [info exists F2C] and $F2C ne ""} {
        HTTP::cookie insert name "F2C" value $F2C
        unset -nocomplain F2C
    }
}
when HTTP_REQUEST_DATA {
    log local0. "Entering Request Data event"
    set refid "unknown"
    set refclickid "unknown"
    foreach x [split [string tolower [HTTP::payload]] "&"] {
        log local0. "x = $x"
        if { $x starts_with "refid=" } {
            set refid [lindex [split $x "="] 1]
            log local0. "refid = $refid"
        } 
        if { $x starts_with "refclickid=" } {
            set refclickid [lindex [split $x "="] 1]
            log local0. "refclickid = $refclickid"
        }   
    }
    HSL::send [HSL::open -proto UDP -pool syslog_server_pool_1] "<190> [IP::client_addr] \[[clock format [clock seconds] -format "%d/%b/%Y:%H:%M:%S %z"]\] --[IP::local_addr]--pbrefid=$refid--pbrefclickid=$refclickid--F2C=$F2C"
}