XML RPC API - Failure, Invalid or null criteria specified

I'm using the XML RPC API's for editing data, following the example that they gave (modified).
This is the XML that I am trying to parse with the API:

<textarea name="XMLString" >
<ZohoCreator>
    <applicationlist>
        <application name='supportv15'>
            <formlist>
                <form name='Main_Database'>
                    <update>
                        <criteria>
                            <![CDATA[(Return_ID=="HORP53376")]]>
                        </criteria>
                        <newvalues>
                            <field name='Name'><value>Name</value></field>
                        </newvalues>
                    </update>
                </form>
            </formlist>
        </application>
    </applicationlist>
</ZohoCreator>
</textarea>

The returned XML will always say
<status>Failure, Invalid or null criteria specified</status>

I've tried multiple different ways of formatting the criteria, but no matter what it will not accept it.

Also, there are 2 RPC API's, one of them seems to be woefully outdated, yet still appears when searching for "Zoho RPC API": https://writer.zoho.com/public/help/creator-api-xml/fullpage
Whereas, the current (and different) one seems to be this: https://www.zoho.com/creator/help/api/xml-rpc-api/xml-rpc-api-edit-records.html