There are many static data resources in the current MQTT library (v1.9.0) that are shared by all clients. the semaphore lock that is connected to the "static MqttProp clientPropStack[MQTT_MAX_PROPS]". This resource ran out while just TWO clients were active, thus as a workaround, the MQTT_MAX_PROPS value had to be raised by multiplying the default value by the number of clients.