JasperReports: xsd for custom component not found

Question

I made a new chart type as custom component for JasperReports. I inserted my custom component into the jrxml but Jaspersoft Studio says it cannot find the xsd:

net.sf.jasperreports.engine.JRRuntimeException: Unknown entity http://jasperreports.sourceforge.net/xsd/blockchart.xsd, not loading.

I checked that the jar file contains blockchart.xsd. And the schema location is defined in the blockchart_beans.xml:

    <bean id="xmlParser" class="net.sf.jasperreports.engine.component.DefaultComponentXmlParser">
        <property name="namespace">
            <value>http://jasperreports.sourceforge.net/jasperreports/blockchart</value>
        </property>
        <property name="publicSchemaLocation">
            <value>http://jasperreports.sourceforge.net/xsd/blockchart.xsd</value>
        </property>
        <property name="internalSchemaResource">
            <value>de/xyz/jasperreports/blockchart/blockchart.xsd</value>
        </property>
        <property name="digesterConfigurer">
            <bean class="de.xyz.jasperreports.blockchart.BlockChartsDigester"/>
        </property>
    </bean>

My jrxml:

                <componentElement>
                    <reportElement x="0" y="0" width="800" height="510" uuid="550e8442-e29b-11d4-a716-446655440042" />
                    <bc:blockChart evaluationTime="Group" evaluationGroup="mygroup"
                      xmlns:bc="http://jasperreports.sourceforge.net/jasperreports/blockchart"
                      xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports/blockchart http://jasperreports.sourceforge.net/xsd/blockchart.xsd">
                        <bc:chartSettings customizerClass="de.xyz.jasper.Report1ChartCustomizer">
                    </bc:blockChart>
                </componentElement>

Why does Jaspersoft Studio not find my xsd? Any idea what I could have missed?