LNK304P unexpected output voltage
This is, in my opinion, a bizarre buck regulator. It's also rather unsafe because it's a non-isolated mains voltage regulator.

If you want to be sure that PC101 is your issue, I would measure PC101 pins 3 and 4 to check if it's shorted. If it is, then you know that's the issue (or at least the main issue). The other thing to check would be to setup a 10mA current between pins 1 and 2 and measure the resistance on pins 3 and 4. If the resistance drops when you power the opto-coupler, then you know it's still working.

If that turns out to be fine, I would check all of the diodes to make sure they're operating as expected. Use a diode checker on them. They should show an open circuit in one direction and virtually short for the other direction. For the zener's you'll also want to test them for their reverse voltage level. To test that, you need to put a voltage supply with a current limiting resistor on them and voltage probe the diode contacts. You should see approximately the reverse breakdown voltage.