Problems with IGBT driving
This is not the greatest of gate-drives.

What you basically have is 4k4 for Turn-ON  (PC817 pulls the gate resistor up to 12V) and 10k for Turn-OFF (a simple gate termination resistor) 

These are huge values and you would normally expect to see TurnOn and TurnOFF resistances in the order of 10's of Ohms.

There is equally no freewheel diode across the load. Even a resistor has inductance and thus force commutating an inductive load without a circulation path will cause voltage spikes (possibly to breakdown if the stray capacitance cannot squash it).

You are probably being saved from any overshoot via the HUGE turnOFF time, essentially using the IGBT as a linear resistor 

What is probably happening is the IGBT has dissipated too much power due to the large switching losses and finally failed.

Advice


Include a freewheel diode for the load
Change the gatedrive topology - still use that OPTO but use a BJT push-pull stage to provide a low impedance to the IGBT