Buzzing, hm? That would localize the issue to the AC-DC conversion, and it sounds like a component failed in that 12V wallwart...which isn't too surprising, since the requirement for 15V is based on a step-down that translates that to the various DC busses and since wallwarts are often horrid little pieces of cheapo crap. But if you power it with 12V, you're pushing the DC-DC converters in the synth's power supply really hard, as that's not the voltage the P/S is expecting. The result is that even if you specced the build for the power capacity of that Palette 62 and had everything added up correctly, the 12V wart is lowering the current available, which also appears to be playing a part here. Betcha if you put that 12V wart on a scope now, you'll see quite a bit of AC getting past the rectification.
Things like this are why bunches of us make a point of saying to NOT overrun the DC specs...which includes the external wallwart needed for the DC busses. Always spec your available current at at least 3/4ths of what the manufacturer says is the maximum current draw to avoid the ugly surprise of inrush currents popping the fuse (and ONLY the fuse...if you're lucky!) or causing other trouble. And that also goes for the Intellijel's onboard DC-DC conversion; if the specs for the Palette 62 says it wants to see a 40 watt wart, put a 60 watt one on it instead. Since the Palette 62 doesn't need the extra 20 watts, what that'll do is to let the bigger wallwart loaf along at 2/3rd's of the maximum draw at the DC end. And loafing P/Ss is what you want, as that reduces the component heat, which makes the wallwart happy and far less susceptible to kicking off due to a current overdraw.