As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE OPERATING THE APU WITH THE NR 2 GENERATOR PLACARD INOPERATIVE, THE APU FLAMED OUT AT FL 310. A RESTART ATTEMPT WAS UNSUCCESSFUL. THE AIRCRAFT WAS RETURNED TO IAH WHERE IT LANDED WITHOUT INCIDENT. MAINTENANCE STARTED THE APU SEVERAL TIMES AND RAN IT FOR ONE HOUR WITH NO PROBLEMS NOTED.