The servers participating in a network of brokers must have a <networkConnections> element in their activemq.xml. Do note that the elements within the <broker> elements *must* appear alphabetically - otherwise the server will fail without notice!
This is the configuation file for instance1 on sandkasse-01:
<?xml version="1.0" encoding="UTF-8"?><beans xmlns="[http://www.springframework.org/schema/beans|http://www.springframework.org/schema/beans]" xmlns:amq="[http://activemq.apache.org/schema/core|http://activemq.apache.org/schema/core]" xmlns:xsi="[http://www.w3.org/2001/XMLSchema-instance|http://www.w3.org/2001/XMLSchema-instance]" xsi:schemaLocation="[http://www.springframework.org/schema/beans|http://www.springframework.org/schema/beans] [http://www.springframework.org/schema/beans/spring-beans-2.0.xsd|http://www.springframework.org/schema/beans/spring-beans-2.0.xsd]   [http://activemq.apache.org/schema/core|http://activemq.apache.org/schema/core] [http://activemq.apache.org/schema/core/activemq-core.xsd|http://activemq.apache.org/schema/core/activemq-core.xsd]">

    <!-- Allows us to use system properties as variables in this configuration file -->
    <bean>
        <property name="locations">
            <value>file:${activemq.base}/conf/credentials.properties</value>
        </property>
    </bean>

    <!--
        The <broker> element is used to configure the ActiveMQ broker.
    -->
    <broker xmlns="[http://activemq.apache.org/schema/core|http://activemq.apache.org/schema/core]" brokerName="instance1" dataDirectory="${activemq.base}/data" destroyApplicationContextOnStop="true">

        <!--
                        For better performances use VM cursor and small memory limit.
                        For more information, see:

            [http://activemq.apache.org/message-cursors.html|http://activemq.apache.org/message-cursors.html]

            Also, if your producer is "hanging", it's probably due to producer flow control.
            For more information, see:
            [http://activemq.apache.org/producer-flow-control.html|http://activemq.apache.org/producer-flow-control.html]
        -->

        <destinationPolicy>
            <policyMap>
              <policyEntries>
                <policyEntry memoryLimit="1mb" producerFlowControl="true" topic="&gt;">
                  <pendingSubscriberPolicy>
                    <vmCursor/>
                  </pendingSubscriberPolicy>
                </policyEntry>
                <policyEntry memoryLimit="1mb" producerFlowControl="true" queue="&gt;">
                  <!-- Use VM cursor for better latency
                       For more information, see:

                       [http://activemq.apache.org/message-cursors.html|http://activemq.apache.org/message-cursors.html]

                  <pendingQueuePolicy>
                    <vmQueueCursor/>
                  </pendingQueuePolicy>
                  -->
                </policyEntry>
              </policyEntries>
            </policyMap>
        </destinationPolicy>

        <!--
            The managementContext is used to configure how ActiveMQ is exposed in
            JMX. By default, ActiveMQ uses the MBean server that is started by
            the JVM. For more information, see:

            [http://activemq.apache.org/jmx.html|http://activemq.apache.org/jmx.html]
        -->
        <managementContext>
            <managementContext createConnector="false"/>
        </managementContext>

        <!--
            Added by csv 20110530:
            The network connectors enumerate other brokers participating in
            a network of brokers
           {color} [http://activemq.apache.org/networks-of-brokers.html|http://activemq.apache.org/networks-of-brokers.html]
        -->

        <networkConnectors>
            <networkConnector name="instance2" uri="static:(tcp://localhost:60002)"/>
            <networkConnector name="instance3" uri="static:(tcp://localhost:60003)"/>
        </networkConnectors>

        <!--
            Configure message persistence for the broker. The default persistence
            mechanism is the KahaDB store (identified by the kahaDB tag).
            For more information, see:

            [http://activemq.apache.org/persistence.html|http://activemq.apache.org/persistence.html]
        -->
        <persistenceAdapter>
            <kahaDB directory="${activemq.base}/data/kahadb"/>
        </persistenceAdapter>

          <!--
            The systemUsage controls the maximum amount of space the broker will
            use before slowing down producers. For more information, see:

            [http://activemq.apache.org/producer-flow-control.html|http://activemq.apache.org/producer-flow-control.html]

        <systemUsage>
            <systemUsage>
                <memoryUsage>
                    <memoryUsage limit="20 mb"/>
                </memoryUsage>
                <storeUsage>
                    <storeUsage limit="1 gb"/>
                </storeUsage>
                <tempUsage>
                    <tempUsage limit="100 mb"/>
                </tempUsage>
            </systemUsage>
        </systemUsage>
                -->

        <!--
            The transport connectors expose ActiveMQ over a given protocol to
            clients and other brokers. For more information, see:

            [http://activemq.apache.org/configuring-transports.html|http://activemq.apache.org/configuring-transports.html]
        -->
        <transportConnectors>
            <transportConnector name="openwire" uri="tcp://0.0.0.0:60001"/>
        </transportConnectors>

    </broker>

    <!--
        Enable web consoles, REST and Ajax APIs and demos
        It also includes Camel (with its web console), see ${ACTIVEMQ_HOME}/conf/camel.xml for more info

        Take a look at ${ACTIVEMQ_HOME}/conf/jetty.xml for more details
    -->
    <import resource="jetty.xml"/>

</beans>