Ah, I think this is a bug in the latest update. Try the following:
chown -R pioreactor:www-data /home/pioreactor/.pioreactor/storage
chmod -R 770 /home/pioreactor/.pioreactor/storage
and then try:
curl localhost/unit_api/calibrations | jq
If you see an error again from that curl command, try pio logs and lmk what you see there.