As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAXI OUT, HAD A LAVATORY SMOKE CAS MESSAGE, 20 SECONDS LATER SMOKE WAS REPORTED IN CABIN. SHUT DOWN PACKS AND NO MORE SMOKE WAS COMING INTO CABIN. RETURNED TO THE GATE AND MAINTENANCE FOUND THAT THE LT HX FAN CIRCUIT BREAKER HAD POPPED. MAINTENANCE RESET THE CIRCUIT BREAKER, GROUND RUN CHECKED GOOD.