DCC decoder - interupt/i2c problem

Dear all,

I have a problem, I found a great sketch on the internet for my model train. The sketch is to make an accesoiry decoder to control the switches and signals. This code works perfectly! The downside is that I have not enough outputs.

So I thought I connect an i2c-IOextender. But here is the problem: the moment I activate the I2C the script stops working :frowning:

I dont understand what is wrong. The only thing I can think of is that the interupt routine is interfering with the I2C on my Atmel328.

Does anyone know how to solve this problem?