As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED RED CABIN ALTITUDE MESSAGE IN FLIGHT. CABIN ALTITUDE REACHED 10,200 FEET. CREW DECLARED AN EMERGENCY, RETURNED TO FIELD AND LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED AND FOUND AFT CARGO DOOR NOT CLOSED PROPERLY. MAINTENANCE CLOSED DOOR AND PERFORMED OPERATIONAL CHECK OF AFT CARGO DOOR IAW AMM. OPERATIONAL CHECK GOOD. NO DEFECTS NOTED. ACFT RELEASED FOR SERVICE.