Upgrading from 1.4.3 to 2.1.3 it's possible you can no longer log in to maxadmin:
Unable to connect to MaxScale at localhost, 6603: Connection refused
(MaxScale is listening on 6603)
Trying to connect via the socket:
$ maxadmin -S /tmp/maxadmin.sock
Could connect to MaxScale, but was not authorized.
Although reading the documentation explains everything and how to solve it, it would be good to have more helpful messages explaining why the login failed.