On several of the TRIACS, it appears that there is little to no solder at all on the middle leg. This probably relates back to the thought about trying to solder the triac while the heat sink is attached. I'd suggest removing the heat sink, touching up the solder joints on the triacs . Test the board without the heat sink attached. If all is well, clean up the back of the board by nipping all of the legs (all components) then reattaching the heat sink.
WHen running with power, recommend that you have the LE sitting on a non-conductive material.
Larry
(edit)... after re-reading the posts here and looking closer at the pics of the back of the board, I found/see some very shiny spots on the board surface between the big ground plane and the pins for the output. Check the board closely for any dollups of solder that should not be.