As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT NOT GROUNDED: AFTER ENGINE START SFO UPON APU SHUTDOWN ANOTHER FLIGHT CREW REPORTED FLAMES /SMOKE COMING FROM APU EXHAUST, ARFF RESPONDED RETURNED TO GATE NO COCKPIT INDICATIOS OF APU FIRE OR ECAM INDICATIONS OF MALFUNCTION. TROUBLESHOT PER TSM 49-00-81. FOUND APU GEARBOX VENT TUBE BLOCKED. CLEANED VENT TUBE AND FITTING IAW AMM 49-26-55.