Can't reconnect to MQTTSN gateway

I have been running a mqttsn client publisher/subscriber project (based on the example) successfully for some months.  I have added a mosquitto broker to the raspberry pi border router.  Each thread node publishes data every 10 seconds.

SDK:   nRF5_SDK_for_Thread_and Zigbee_2.0.0

RaspPi image:   RaspPi_Thread_Border_Router_Demo_2.0.0-1.alpha

I turned both the raspberry pi and thread node on at the same time several weeks ago.  Today, the connection to the broker was lost and cannot be regained.  A second thread node with debugger attached shows that the gateway is found, but the connection ( mqttsn_client_connect(&m_client, &m_gateway_addr, m_gateway_id, &m_connect_opt); ) times out repeatedly.   I am assuming the Thread network connection is ok, since the gateway was found.

My PC remains connected to the broker on the raspberry pi, but is not receiving any messages.  I have repeatedly restarted the node attached to the debugger, and it continues to timeout on the mqtt gateway connection.

This error occurs infrequently (only after running continuously for several weeks).  What can I check on the raspberry pi to diagnose the problem?