Virtual server with two different web servers

You can try this and see what the logs turn up... Maybe that will help figure something out... Are you familiar with tcpdump too?

when HTTP_REQUEST {
    log local0. "URL: [HTTP::host][HTTP::uri]"
}
when HTTP_RESPONSE { 
    log local0. "  Response code: [HTTP::status]"
    if { [HTTP::is_redirect] }{ 
        log local0. "    Original: [HTTP::header value Location]"
        HTTP::header replace Location [string map {"A.internal.com" "X.external.com"} [HTTP::header Location]] 
        log local0. "    Updated: [HTTP::header value Location]"
    } 
}