System notice 'Connection failed to broker'

Hi there,
I am building a vertical farm sensing and control system.
The sensor data are gathered into my node-red server through MQTT server (Mosquitto).

Recently, console alert 'connection failed to broker' messages pops up frequently.
And MQTT communication published from sensor fails to connect,
however, publishing to sensor from server seems fine.

Why this problem has occured?
I attached some error message below. Please give some advice!

I guess the problem is because of MQTT Connection limit...
How many MQTT Node Connections are possible?
System has a lot of connection depended one MQTT node-server. Does this cause the problem?

please don't double post - stick to one thread.

Closing this duplicate.