Serializer for JacksonJsonNode not found


I’m trying to serialize large collections of data during the execution of my processes by utilizing the Camunda Spin library.
My goal was to map those collections to SpinJsonNode’s and just send them as BLOBs with the completion of my external tasks as stated here and here, since the DB imposes a 4k character limit for string serialization.

My server throws the following exception when trying to persist the BLOBS

org.camunda.bpm.client.exception.NotResumedException: TASK/CLIENT-01009 Exception while completing the external task: The corresponding process instance could not be resumed. Reason: status code: 500, reason phrase: {"type":"ProcessEngineException","message":"Cannot find serializer for value 'ObjectValue [value=null, isDeserialized=false, serializationDataFormat=application/json, objectTypeName=org.camunda.spin.impl.json.jackson.JacksonJsonNode, serializedValue=157 chars, isTransient=false]'."}

The logic for mapping objects to JSON and returning the result map that gets sent during completion looks like this

 private Map<String, Object> createResultMapFromObject(Object o) {
        Map<String, Object> resultMap = new HashMap<>();
        resultMap.put("demoList", JSON(o));
        return resultMap;

Accordingly the logic that maps Jackson BLOBs to ArrayLists of Strings looks like this:

private List<String> fetchList(ExternalTask externalTask) {
        SpinJsonNode jsonNode = (SpinJsonNode) externalTask.getVariable("demoList");
        if (jsonNode == null) {
            return new ArrayList<>();
        return jsonNode.mapTo("java.util.ArrayList<java.lang.String>");

But as you can see the code fails at the serialization step and I never had the chance to test if the deserialization works with non mocked components.

I appreciate help and am looking forward to answers and suggestions
Best Regards