As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
EICAS WARNING CAUTION MSG STARTING WITH RT AC BUSS OFF WITH BOTH GENERATORS WORKING NORMALLY AND NO FURTHER STATUS MSGS AND PROPER VOLTS AND FREQS. BITE ON BPCU FOUND NO FAULTS. DEPOWERED ACFT, RESTORED PWR. MSG CLEARED. STARTED BOTH ENGINES AT GATE MSG DID NOT RETURN. ALL INDICATIONS NORMAL. ACFT OK FOR SV. ETOPS CATEGORY 'E': PROBLEMS WITH SYS CRITICAL TO ETOPS.