I see a similar problem was reported as V1.10.0 Failed to set client configurations.
In that thread the question How have you created the SSL certificate for your MQTT server? was asked. In my case the certificate was created using Let’s Encrypt. This results in two files: fullchain.pem and privkey.pem. I have further simplified the mosquitto configuration file to:
/etc/mosquitto/conf.d/ruuvigateway.conf
protocol mqtt
listener 8883
bind_interface eno1
certfile /etc/mosquitto/certs/fullchain.pem
keyfile /etc/mosquitto/certs/privkey.pem
password_file /etc/mosquitto/passwd
Which works using mosquitto_pub with no --capath or --cafile options.