As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: APU WAS RUNNING IN FLIGHT DUE TO MEL REQUIREMENT. APPROXIMATELY 10 MINUTES AFTER LEVEL OFF WHILE CRUISING AT FL 390, APU AMBER LOW OIL PRESSURE LIGHT ILLUMINATED WITH ASSOCIATED MASTER CAUTION. QRH PROCEDURE ACCOMPLISHED. UPON CONSULTATION WITH DISPATCH, RETURN TO GEG WAS RECOMMENDED DUE TO SINGLE GENERATOR OPERATION. (OPEN)