cqwp : problem with QueryOverride

Hello,

In a CQWP (code snippet):

 <property name="QueryOverride" type="string" ><![CDATA[<Where> <Eq> <FieldRef Name='Thematologia' /><Value Type='Lookup'>EME</Value></Eq></Where><OrderBy><FieldRef Name='ContentDate' Ascending='False' /> </OrderBy> ]]></property>
        <property name="CommonViewFields" type="string">Thematologia,Lookup;ContentDate,DateTime</property>
        <property name="AdditionalGroupAndSortFields" type="string">Thematologia,ContentDate</property>
        <property name="AdditionalFilterFields" type="string">ContentDate,Thematologia</property>

An error arises ("There is a problem with the query that this Web Part is issuing.....") . I deleted the QueryOverride data and the error disappeared. Could you please tell me which is the error? 

Thank you

Christos