Magento 2 Message Queue - Type Error with expected string, null given

First of all, I am not new to Magento 2 development, I know how to create modules. However, I struggle for 3 days now to get a simple Message Queue in Magento 2 to work. Here are my XML files:

communication.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:Communication/etc/communication.xsd">
    <topic name="erp.queue.order"
           request="string">
        <handler name="erp.queue.order"
                 type="TimoG\OrderTransfer\Model\Queue\Consumer"
                 method="process" />
    </topic>
</config>

queue_consumer.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework-message-queue:etc/consumer.xsd">
    <consumer name="erp.queue.order"
              queue="erp.queue.order"
              connection="db"
              handler="TimoG\OrderTransfer\Model\Queue\Consumer::process"/>
</config>

queue_publisher.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework-message-queue:etc/publisher.xsd">
    <publisher topic="erp.queue.order">
        <connection name="db"
                    exchange="magento-db"
                    disabled="false"/>
    </publisher>
</config>

queue_topology.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework-message-queue:etc/topology.xsd">
    <exchange name="magento-db"
              type="topic"
              connection="db">
        <binding id="TimoGErpOrderTransfer"
                 topic="erp.queue.order"
                 destinationType="queue"
                 destination="erp.queue.order"/>
    </exchange>
</config>

So adding messages to the queue works (I see it in the database table "queue_message"), however when I execute php bin/magento queue:consumers:start erp.queue.order I always get the following exception:

Type Error occurred when creating object: Magento\Framework\MessageQueue\Consumer\Config\Data, Argume
  nt 2 passed to Magento\Framework\Reflection\TypeProcessor::resolveFullyQualifiedClassName() must be o
  f the type string, null given, called in /Users/timog/Sites/magento2.local/vendor/magento/fra
  mework/Reflection/TypeProcessor.php on line 545

My consumer test code:

<?php

namespace TimoG\OrderTransfer\Model\Queue;

use Exception;
use Psr\Log\LoggerInterface;
use Magento\Framework\Serialize\Serializer\Json;

class Consumer
{

    private $_logger;
    private Json $_json;


    public function __construct(
        LoggerInterface $logger,
        Json $json
    )
    {
        $this->_json = $json;
        $this->_logger = $logger;
    }


    public function process($request)
    {
        try {
            $test = $this->_json->unserialize($request);
        } catch (Exception $exc) {
            $this->_logger->critical($exc->getMessage());
        }
    }
}

I have no idea what's wrong and I really need help please... it's frustrating...

Thank you!