See https://forum.pfsense.org/index.php?topic=92437.0

Can I ask how you got the XML dump? Did you just do a capture or is there a way to get the output elsewhere?

I have exactly the same issue since the upgrade in the same configuration as you.

Sending to backup :

[2.2.2-RELEASE][admin@ZNA]/etc: ./rc.filter_synchronize
May  6 11:05:16 ZNA php: rc.filter_synchronize: Beginning XMLRPC sync to https://10.0.0.203:443.
May  6 11:05:17 ZNA php: rc.filter_synchronize: XMLRPC sync successfully completed with https://10.0.0.203:443.

---SENT---
POST /xmlrpc.php HTTP/1.0
User-Agent: PEAR XML_RPC
Host: 10.0.0.203
Authorization: Basic my_hashed_password_was_here
Content-Type: text/xml
Content-Length: 203

 <methodcall><methodname>pfsense.filter_configure</methodname>
 <params><param>
<value><string>my_password_was_here</string></value></params></methodcall> 

---END---

Answer I got back:

---GOT---
HTTP/1.0 200 OK
Expires: Fri, 08 May 2015 11:05:18 GMT
Cache-Control: max-age=180000
Content-type: text/html
Connection: close
Date: Wed, 06 May 2015 09:05:19 GMT
Server: lighttpd/1.4.35

Warning: in_array() expects parameter 2 to be array, null given in /etc/inc/unbound.inc on line 239

Warning: in_array() expects parameter 2 to be array, null given in /etc/inc/unbound.inc on line 239

Warning: in_array() expects parameter 2 to be array, null given in /etc/inc/unbound.inc on line 239

Warning: Cannot modify header information - headers already sent by (output started at /etc/inc/unbound.inc:239) in /etc/inc/xmlrpc_server.inc on line 431

Warning: Cannot modify header information - headers already sent by (output started at /etc/inc/unbound.inc:239) in /etc/inc/xmlrpc_server.inc on line 431

 <methodresponse><params><param>
<value><boolean>1</boolean></value></params></methodresponse> 
---END---

May  6 11:05:26 ZNA php: rc.filter_synchronize: An error code was received while attempting Filter sync with username admin https://10.0.0.203:443 - Code 2: Invalid return payload: enable debugging to examine incoming payload
May  6 11:05:26 ZNA php: rc.filter_synchronize: New alert found: An error code was received while attempting Filter sync with username admin https://10.0.0.203:443 - Code 2: Invalid return payload: enable debugging to examine incoming payload

/etc/inc/unbound.inc

        if (isset($config['unbound']['forwarding'])) {
                $dnsservers = array();
                if (isset($config['system']['dnsallowoverride'])) {
                        $ns = array_unique(get_nameservers());
                        foreach($ns as $nameserver) {
                                if ($nameserver)
                                        $dnsservers[] = $nameserver;
                        }
                }
                $sys_dnsservers = array_unique(get_dns_servers());
                foreach ($sys_dnsservers as $sys_dnsserver) {
239 -->                   if ($sys_dnsserver && (!in_array($sys_dnsserver, $ns))) {
                                $dnsservers[] = $sys_dnsserver;
                        }
                }

Seems that $ns is null when you don't have DNS Allow Override (I guess this is when you check the box that states that your DHCP interfaces can override your manually set DNS.

We probably miss a $ns = array(); in line 229 in the slave to prevent him to send PHP warnings that are not XML compliant indeed.

@patch

229a230
>               $ns = array();

I can't log a redmine issue :(

Hope that helps.