Burnt switchboard
From the NHK English link that stopped working on the previous page. Japanese NHK pre-translation original also available if requested. In the quest for detail:
"...the switchboard on a 787 jet was blackened, apparently by heat, during a pilot training flight from Tokyo to Hokkaido on May 4th. It says a message indicating a problem with the jet's electric system appeared on one of the cockpit screens, but no smoke was detected. The cockpit crew used a backup system to continue the flight before the plane landed safely at an airport in Chitose, Hokkaido. A close check by maintenance staff showed part of the board in the electrical room beneath the middle of the cabin had been blackened. One of the nuts that connects wires and the board had come loose and an area around it of about 4 square centimeters had turned black. ANA suspect that staff from Boeing had failed to tighten the nut after they loosened it during pre-flight checks, causing the nut and its surrounding area to overheat."