On a Pi running the latest raspbian disto 4.14.34-v7+ dated Apr 16, 2018, the mosquitto version installed from the repos is 1.4.10. The latest version on Download | Eclipse Mosquitto is 1.4.15.
2.1.15 does not appear to be a valid mosquitto version. It might be a repackaging of mosquitto with its own version number. I would try installing mosquitto from the repos.
Or enable verbose/debug logging in the /etc/mosquitto/mosquitto.conf file then examine the mosquitto log file. There may be helpful messages explaining why clients are failing to connect.
Or use wireshark/tcpdump to capture and examine MQTT packets.