[Kimura and Sawada (Hokuto)]

 The leak at the IXC cross pipe flange (+Y side) was caused by forgetting to remove the tape that temporarily secures the elastomer seal.

 (See attached photos 1~6)
 The forgotten tape was compressed by the flange, and it is presumed that the leak occurred through a small gap created here.
The tape was removed and a new elastomer seal was attached to the gasket sealing surface to close the flange.
The claw clamps closing the flange (36 in total) was tightened diagonally with torques of 20, 40, and 60 Nm.