Circuit won't function on a unique network

I have come upon an interesting problem with my Arduino; I build a circuit and write code that compiles OK, however it will not run on any of the computers on my router - 4 different computers on my home network. Yet, if I take the identical setup to a friend's home, it runs flawlessly. This happens whether the Arduino is a Genuino or a Chinese clone.

The problem has just started . Previous builds ran fine but something has entered my home network that interferes with the process.

Has anyone out there encountered this problem?

What happens when you go back to a previous build? Does it work again?

When going to your friend, do you use it as is or do you compile/upload again on his/her system?

Your description does not give any details except that you use and Arduino (which one), a circuit (which one, schematic please) and run code (please provide).