It looks like the larger NON electrolytic cap might have shorted.
OR
The "lamp change" switch was making poor contact, and over heated the board and surrounding components.
The larger blue cap is part of the power supply and the smaller blue cap is part of the PLC coupling network from the 120Khz transformer.
I'm sticking with my "power supply reactor cap shorting" guess. If the switch was standard vertical mount in a wall box I think if it was the power disconnect bar switch over heating, you would have more damage to the top of the switch, not two burn spots on back and side. $0.02. Unfortunately I would have preferred it be the switch causing the problem....
One has to wonder what would cause a non electrolytic, mylar cap, in service for almost two years, to short. Too bad the damage is too severe to see the original markings on the cap.
When you say power supply reactor cap, do you mean the 220uf electrolytic? IF that shorted, the only thing that would happen is the module wouldn't work. IF it opened, same thing... no workee.
The 22 ohm resistor MAY cook but the V- is about 18volts dc, and another 10% drop across the 22 ohm shouldn't make the resistor get that much warmer (even though it runs hot as is, probably why they went with 2 10 in series, to reduce the power dissipation on the resistors).
But we will never know exactly what happened... too bad!
BTW.... are you SURE that ONLY two 40watt incandescent lamps were on the dimmer??? no CFL's or ceiling fan?
Those are reactive loads and COULD create higher voltage spikes on the load side of the triac that COULD damage things.
I have many old style wall switches for years, and have (gasp) run transformers and tried cfl's on them, and have never had this problem.
The ONLY time I cooked an old style appliance module is when I had a floating neutral, and the line voltage went WAY over 120.... popped the varistor in the module due to overvoltage.... THAT was EXCITING and spectacular!!!