Hello @Nele - I am facing the same error in almost the same situation. Here is my process model:
Its simplest of the process and here is the model xml:
<?xml version="1.0" encoding="UTF-8"?>
<bpmn:definitions xmlns:bpmn="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:dc="http://www.omg.org/spec/DD/20100524/DC" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:camunda="http://camunda.org/schema/1.0/bpmn" xmlns:di="http://www.omg.org/spec/DD/20100524/DI" id="Definitions_031j0vs" targetNamespace="http://bpmn.io/schema/bpmn" exporter="Camunda Modeler" exporterVersion="4.5.0">
<bpmn:process id="cloudtalk_decrement_order" name="cloudtalk_decrement_order" isExecutable="true">
<bpmn:startEvent id="StartEvent_start_decrement_order" name="Start Decrement Order">
<bpmn:outgoing>Flow_0rqaadl</bpmn:outgoing>
</bpmn:startEvent>
<bpmn:sequenceFlow id="Flow_0rqaadl" sourceRef="StartEvent_start_decrement_order" targetRef="Activity_1h060fa" />
<bpmn:endEvent id="Event_end_decrement_order" name="End Decrement Order">
<bpmn:incoming>Flow_0shsr9o</bpmn:incoming>
</bpmn:endEvent>
<bpmn:serviceTask id="Activity_1h060fa" name="1. Get All Subscribers of the Customer" camunda:asyncBefore="true" camunda:delegateExpression="${searchSubscribersDelegate}">
<bpmn:extensionElements>
<camunda:failedJobRetryTimeCycle>R5/PT30S</camunda:failedJobRetryTimeCycle>
</bpmn:extensionElements>
<bpmn:incoming>Flow_0rqaadl</bpmn:incoming>
<bpmn:outgoing>Flow_0rj4hqr</bpmn:outgoing>
</bpmn:serviceTask>
<bpmn:serviceTask id="Activity_01ya1tz" name="2. Get Subscriber Details" camunda:asyncBefore="true" camunda:delegateExpression="${searchSubscriberDetailsDelegate}">
<bpmn:extensionElements>
<camunda:failedJobRetryTimeCycle>R5/PT30S</camunda:failedJobRetryTimeCycle>
</bpmn:extensionElements>
<bpmn:incoming>Flow_0rj4hqr</bpmn:incoming>
<bpmn:outgoing>Flow_1ile4eg</bpmn:outgoing>
</bpmn:serviceTask>
<bpmn:serviceTask id="Activity_1fslz4p" name="3. Delete Subscribers" camunda:asyncBefore="true" camunda:delegateExpression="${deleteSubscribersDelegate}">
<bpmn:extensionElements>
<camunda:failedJobRetryTimeCycle>R5/PT30S</camunda:failedJobRetryTimeCycle>
</bpmn:extensionElements>
<bpmn:incoming>Flow_1ile4eg</bpmn:incoming>
<bpmn:outgoing>Flow_0qrbh9l</bpmn:outgoing>
</bpmn:serviceTask>
<bpmn:serviceTask id="Activity_01fyrqr" name="4. Decrement Order" camunda:asyncBefore="true" camunda:delegateExpression="${decrementOrderDelegate}">
<bpmn:extensionElements>
<camunda:failedJobRetryTimeCycle>R5/PT30S</camunda:failedJobRetryTimeCycle>
</bpmn:extensionElements>
<bpmn:incoming>Flow_0qrbh9l</bpmn:incoming>
<bpmn:outgoing>Flow_0shsr9o</bpmn:outgoing>
</bpmn:serviceTask>
<bpmn:sequenceFlow id="Flow_0shsr9o" sourceRef="Activity_01fyrqr" targetRef="Event_end_decrement_order" />
<bpmn:sequenceFlow id="Flow_0rj4hqr" sourceRef="Activity_1h060fa" targetRef="Activity_01ya1tz" />
<bpmn:sequenceFlow id="Flow_1ile4eg" sourceRef="Activity_01ya1tz" targetRef="Activity_1fslz4p" />
<bpmn:sequenceFlow id="Flow_0qrbh9l" sourceRef="Activity_1fslz4p" targetRef="Activity_01fyrqr" />
</bpmn:process>
<bpmndi:BPMNDiagram id="BPMNDiagram_1">
<bpmndi:BPMNPlane id="BPMNPlane_1" bpmnElement="cloudtalk_decrement_order">
<bpmndi:BPMNEdge id="Flow_0shsr9o_di" bpmnElement="Flow_0shsr9o">
<di:waypoint x="990" y="120" />
<di:waypoint x="1102" y="120" />
</bpmndi:BPMNEdge>
<bpmndi:BPMNEdge id="Flow_0rqaadl_di" bpmnElement="Flow_0rqaadl">
<di:waypoint x="218" y="120" />
<di:waypoint x="290" y="120" />
</bpmndi:BPMNEdge>
<bpmndi:BPMNEdge id="Flow_0rj4hqr_di" bpmnElement="Flow_0rj4hqr">
<di:waypoint x="390" y="120" />
<di:waypoint x="490" y="120" />
</bpmndi:BPMNEdge>
<bpmndi:BPMNEdge id="Flow_1ile4eg_di" bpmnElement="Flow_1ile4eg">
<di:waypoint x="590" y="120" />
<di:waypoint x="690" y="120" />
</bpmndi:BPMNEdge>
<bpmndi:BPMNEdge id="Flow_0qrbh9l_di" bpmnElement="Flow_0qrbh9l">
<di:waypoint x="790" y="120" />
<di:waypoint x="890" y="120" />
</bpmndi:BPMNEdge>
<bpmndi:BPMNShape id="_BPMNShape_StartEvent_2" bpmnElement="StartEvent_start_decrement_order">
<dc:Bounds x="182" y="102" width="36" height="36" />
<bpmndi:BPMNLabel>
<dc:Bounds x="160" y="145" width="82" height="27" />
</bpmndi:BPMNLabel>
</bpmndi:BPMNShape>
<bpmndi:BPMNShape id="Activity_0zygmlt_di" bpmnElement="Activity_1h060fa">
<dc:Bounds x="290" y="80" width="100" height="80" />
</bpmndi:BPMNShape>
<bpmndi:BPMNShape id="Activity_1ja4bub_di" bpmnElement="Activity_01ya1tz">
<dc:Bounds x="490" y="80" width="100" height="80" />
</bpmndi:BPMNShape>
<bpmndi:BPMNShape id="Activity_1ve29z2_di" bpmnElement="Activity_1fslz4p">
<dc:Bounds x="690" y="80" width="100" height="80" />
</bpmndi:BPMNShape>
<bpmndi:BPMNShape id="Activity_1dczydp_di" bpmnElement="Activity_01fyrqr">
<dc:Bounds x="890" y="80" width="100" height="80" />
</bpmndi:BPMNShape>
<bpmndi:BPMNShape id="Event_0uo1y6q_di" bpmnElement="Event_end_decrement_order">
<dc:Bounds x="1102" y="102" width="36" height="36" />
<bpmndi:BPMNLabel>
<dc:Bounds x="1082" y="145" width="78" height="27" />
</bpmndi:BPMNLabel>
</bpmndi:BPMNShape>
</bpmndi:BPMNPlane>
</bpmndi:BPMNDiagram>
</bpmn:definitions>