Optoisolator always on on the high-voltage side
Gate voltage on the BT136 triac is 5V and you are feeding it through a 1k resistor from the live side of the supply. Have you checked the power rating of the 1k resistor? 

You shouldn't have to "dispence" much more than 30mA (typically) for the triac to trigger and this would imply, from a 230V supply, a resistor value of about 7.5k. Even so... it would have to be rated at nearly 7W. A 1k resistor would be a 50W.

OK... it only supplies this power when switching on but I come back to the 1k resistor possibly being broken or blown.

As for the 230V on both sides of the opto -if the BT136 triac is bust this could easily happen.