MQQT Topic crash

I've 3 ESP connect to 1 MQTT broker, ESP 1 running well since 2016. A week ago, I installed ESP 2 and ESP 3 to connect on same MQTT broker, each of them publish a different topic but same structure (eg ESP 2 : "ESP/livingroom/light", ESP 3: "ESP/kitchen/light". The problem is, when ESP 2 & ESP 3 connected to wifi, each of them crash (looping Callback update with Attempting connection to MQQT). But ESP 1 still running well. Is the problem located in coding, Server or MQQT Broker? Many thanks for your helps.