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arrrrg - found the mistake (of course one minute after my post). Here is the correct code:

<?xml version="1.0" encoding="UTF-8"?>
<bpmn:definitions id="ServiceTaskJava"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://schema.omg.org/spec/BPMN/2.0 BPMN20.xsd"
    xmlns:bpmn="http://schema.omg.org/spec/BPMN/2.0" typeLanguage="http://www.w3.org/2001/XMLSchema"
    expressionLanguage="http://www.w3.org/1999/XPath" targetNamespace="http://sample.bpmn.camunda.com/"
    xmlns:jbpm="http://jbpm.org/4.0/bpmn2">


    <bpmn:itemDefinition id="i1">
        <jbpm:arg>
            <jbpm:object expr="#{instanceId}" />
        </jbpm:arg>
    </bpmn:itemDefinition>

    <bpmn:itemDefinition id="i2">
        <jbpm:var name="v1" />
    </bpmn:itemDefinition>

    <bpmn:message id="im" name="input message"
        structureRef="i1"></bpmn:message>

    <bpmn:message id="om" name="output message"
        structureRef="i2">
    </bpmn:message>

    <bpmn:interface id="if1" name="mocks.Mock01">
        <bpmn:operation id="o1" name="call">
            <bpmn:inMessageRef>im</bpmn:inMessageRef>
            <bpmn:outMessageRef>om</bpmn:outMessageRef>
        </bpmn:operation>
    </bpmn:interface>


    <!-- process definition -->
    <bpmn:process id="foobar" name="async_process">

        <bpmn:startEvent id="s" />
        
        <bpmn:sequenceFlow id="f1" name="s_2_r1" sourceRef="s" targetRef="r1" />

        <bpmn:receiveTask id="r1" name="r1" />
        
        <!-- -->
        <bpmn:sequenceFlow id="f2" name="r1_2_t1" sourceRef="r1" targetRef="st1" />

        <bpmn:serviceTask id="st1" name="st1" implementation="Other" operationRef="o1"/>
        
        <bpmn:sequenceFlow id="f3" name="st1_2_r2" sourceRef="st1" targetRef="r2" />
        <!-- -->
        
        <!-- 
        <bpmn:sequenceFlow id="fx" name="r1_2_r2" sourceRef="r1" targetRef="r2" />
        -->
        
        <bpmn:receiveTask id="r2" name="r2" />
        
        <bpmn:sequenceFlow id="f4" name="r2_2_e1" sourceRef="r2" targetRef="e1" />

        <bpmn:endEvent id="e1" name="e1" />            
    </bpmn:process>
</bpmn:definitions>

The problem was the in and out message ref.

Best regards - claus