As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON INTIAL CLIMB LEFT PACK OVER PRESS (C)_ MSG WHEN TRANSFERING BLEEDS C/W QRH MSG REMAINED DURING INITIAL CLIMB OUT OF IAD, WITH THE RH CONDITIONING PACK ALREADY ON MEL, THE AIRCRAFT LOST THE CAPABILITIES OF THE LH PACK. THE CREW DECIDED TO AIR RETURN TO IAD.