Low memory available error

[cent89]

Hi, Your code is very useful, thanks!
However, I have a problem. With Arduino Nano and Arduino Uno, during compiling, I have a warning like "Low memory available, stability problems may occur"; Arduino, after a few minutes of work, crashes and I have to reset.
I found the code in this page: http://tmrh20.github.io/RF24Ethernet/mqtt_basic_8ino-example.html

It's a code problem or, like the waring says, a arduino/memory problem?

Thanks!
Roberto