As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) DURING CLIMB, IT WAS OBSERVED THAT CABIN PRESSURE WAS NOT CONSISTENT WITH FLIGHT LEVEL. AIRCRAFT RETURNED TO BASE WITHOUT INCIDENT. AIRCRAFT WAS DISPATCHED UND ER MEL 21-01-01 WITH RT PACK INOP FLYING AT AN ALTITUDE OF FL 25,000 FT. THE ISOLATION VALVE WAS HARD FAULTED. NO FURTHER INCIDENT DURING SUBSEQUENT FLIGHT.