error 400 bad request with XEP-0055?

I got possible fields from my server:

<iq xmlns="jabber:client" from="vjud.company.com" to="testuser@company.com/iPhone" id="search1" type="result"><query xmlns="jabber:iq:search">

<instructions>You need an x:data capable client to search</instructions>
<x xmlns="jabber:x:data" type="form">
<title>Search users in vjud.company.com</title>
<instructions>Fill in the form to search for any matching Jabber User (Add * to the end of field to match substring)
</instructions>
<field type="text-single" label="User" var="user"/>
<field type="text-single" label="Full Name" var="fn"/>
<field type="text-single" label="Name" var="first"/>
<field type="text-single" label="Middle Name" var="middle"/>
<field type="text-single" label="Family Name" var="last"/>
<field type="text-single" label="Nickname" var="nick"/>
<field type="text-single" label="Birthday" var="bday"/>
<field type="text-single" label="Country" var="ctry"/>
<field type="text-single" label="City" var="locality"/>
<field type="text-single" label="Email" var="email"/>
<field type="text-single" label="Organization Name" var="orgname"/>
<field type="text-single" label="Organization Unit" var="orgunit"/>
</x>
</query>
</iq>

Suppose I want to search a user with JID admin@company.com

Composed request wil look like this:

XMPPIQ *iq2 = [[XMPPIQ alloc] init];
[iq2 addAttributeWithName:@"type" stringValue:@"set"];
[iq2 addAttributeWithName:@"from" stringValue:@"testuser@company.com"];
[iq2 addAttributeWithName:@"to" stringValue:@"vjud.company.com"];
[iq2 addAttributeWithName:@"id" stringValue:@"search1"];
XMPPElement *query2 = [XMPPElement elementWithName:@"query"];
[query2 setXmlns:@"jabber:iq:search"];
XMPPElement *user = [XMPPElement elementWithName:@"user"];
[user setStringValue:@"admin"];
[iq2 addChild:query2];
[query addChild:user];

The error stanza:

<iq xmlns="jabber:client" from="vjud.company.com" to="testuser@company.com/iPhone" type="error" id="search1">
<query xmlns="jabber:iq:search"/>
<error code="400" type="modify">
<bad-request xmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/>
</error>
</iq>

So, basically there are 2 questions:

Why there is no </query> element in response?
Is it the actual reason of the error?

-The server's response should look like this-

Big thanks to @legoscia user, in case request was composed right, you will get something like this(notice <item> element):

   <iq xmlns="jabber:client" from="vjud.company.com" to="testuser@company.com/iPhone" id="search1" type="result»>

 <query xmlns="jabber:iq:search"><x xmlns="jabber:x:data" type="result»>
 <title>Search Results for vjud.company.com</title>
 <reported>
 <field type="text-single" label="Jabber ID" var="jid»/>
 <field type="text-single" label="Full Name" var="fn»/>
 <field type="text-single" label="Name" var="first»/>
 <field type="text-single" label="Middle Name" var="middle»/>
 <field type="text-single" label="Family Name" var="last"/><field type="text-single" label="Nickname" var="nick»/>
 <field type="text-single" label="Birthday" var="bday»/>
 <field type="text-single" label="Country" var="ctry»/>
 <field type="text-single" label="City" var="locality»/>
 <field type="text-single" label="Email" var="email»/>
 <field type="text-single" label="Organization Name" var="orgname"/><field type="text-single" label="Organization Unit" var="orgunit»/>
 </reported>
 <item>
 <field var="jid»>
 <value>admin@company.com</value>
 </field>
 <field var="fn"><value/></field>
 <field var="last"><value/></field>
 <field var="first"><value/></field>
 <field var="middle"><value/></field>
 <field var="nick"><value/></field>
 <field var="bday"><value/></field>
 <field var="ctry"><value/></field>
 <field var="locality"><value/></field>
 <field var="email"><value/></field>
 <field var="orgname"><value/></field>
 <field var="orgunit"><value/></field></item>
 </x>
 </query>
 </iq>

If there is no matches,you will just receive <reported> element with lots of fields. You may want to have look at this as well.

This search service doesn't support "plain" XEP-0055 search fields, but requires you to submit the x:data form returned in the response to the "get" request; see XEP-0004.

You can tell this by the fact that the result doesn't contain any suggested search fields as children of the query element (see example 2 of XEP-0055), and also the <instructions/> element says so. This means that you need to look at the <x xmlns="jabber:x:data" type="form"> element and submit it.

Thus, you need to send something like:

<iq type='set'
    from='juliet@capulet.com/balcony'
    to='characters.shakespeare.lit'
    id='search4'
    xml:lang='en'>
  <query xmlns='jabber:iq:search'>
    <x xmlns='jabber:x:data' type='submit'>
      <field var='user'>
        <value>admin</value>
      </field>
    </x>
  </query>
</iq>

(This is example 8 from XEP-0055, modified to search for the user with username "admin".)