As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED. ONCE WE STARTED BLEEDING APU ON ARRIVAL WE GOT A STRONG ACIDIC SMELL AND SLIGHT SMOKE HAZE IN CABIN. SEEMED TO DISSIPATE WHEN APU BLEED WAS TURNED OFF. HEAVY RAIN ON GATE ARRIVAL MAINTENANCE SERVICED APU PER JOB CARDS 26L00021002 AND 26L00021004.