Blown Capacitor and Bad Zener Diode in a Switching Power Circuit
You should put a scope on the node between the ZD1 cathode and the ++ end of C101. Monitor what happens there under all conditions of low and high range of the VCC/340 node voltage range. For each input condition also vary the current loads on the PANEL_12V and the VDD_15V from the minimum expected levels to the maximum. Also at each of the four corners testing switch the supply on and off a few times. 

You will likely to find conditions where ZD1 is being asked to dissipate significant power or voltage spikes or voltage levels over the rating of the C101 capacitor.