From https://issues.apache.org/jira/browse/AMQ-6526 :

When sending a Stomp message whose payload is bigger than wireFormat.maxDataLength, it results in this error

Exception occurred processing: <Unknown> -> org.apache.activemq.transport.stomp.ProtocolException: The maximum data length was exceeded

The problem is the <Unknown> which should get replaced with the proper Stomp operation that was used.