also suspecting relay, could perhaps moisture inside, mechanical wear/failure, be the cause, i agree it goes back to the relay since you can see the flame starts there, and relays are notorious for sparks arcs, and inductive kicks.\\
i cannot tell furshure but are those 2 puncture holes on top of relay?
... another suspect would be whatever is in the bottom corner of the board since it looks like most of the flame hit there, from here it looks like it almost burned through, also the big black thing next to the big blue thing(the relay i presume), looks as if the flame was traveling up along the outside of the bottom, thus meaning the point of ignition was exterior to the relay, relay is probably rated ok though(not that i checked, i just trust)
pulling this up, i have a feeling its yours
**broken link removed**
judging by alignment with flare i'm now more to say that it was a fault on the main prongs which caused arc then ignition, and fuel: surface looks clean so i doubt grease/oil or dirt buildup, however the plastic could have burned as fuel if it was heated enough, which suggests you had current surge, which indicates somehow a short circuit, and not likely a bad relay.
more likely the prongs or the plug was the failure... maybe a heat buildup in atmosphere to decrease resistance(?)
.....btw im not a pro at anything, this is just my guess