As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SHORTLY AFTER TAKEOFF, THE AIRCRAFT EXPERIENCED A LOUD BANG AND RIGHT DUCT LEAK EICAS MESSAGE. THE AIRCRAFT RETURNED TO FIELD. THE INTERCONNECT DUCT BETWEEN THE LEFT AND RIGHT AIR CONDITIONING PACKS RUPTURED AT A PRODUCTION SEAM. HYDROGEN EMBRITTLEMENT AT THE SEAM WELD OF THE DUCT SEGMENT WAS THE CAUSE OF FAILURE. THE DUCT WAS REPLACED. OPERATIONS CHECK NORMAL.