trying to capture 5XX HTTP response and retry to another pool member - passive monitor

I would probably start with some additional logging:

when HTTP_REQUEST {
    log local0. "Request URI: [HTTP::uri]"
    set http_request [HTTP::request]
}
when HTTP_RESPONSE {
    log local0. "Response status: [HTTP::status]"
    if { ( [HTTP::status] starts_with "5" ) } {
        ...
    }
}

This will at least prove that the HTTP events are being triggered and what they're being triggered for.