When you define both socket and port for a listener you may get the following message:
2018-02-27 13:00:00 error : Listener 'Galera-Listener', for service 'Galera-Service', already have port 7890.
And MaxScale will shut down.
The actual problem should be that both connection methods are defined:
The message is misleading so I'd suggest to use a more adequate one like:
2018-02-27 13:00:00 error : Listener 'Galera-Listener', for service 'Galera-Service', Port method already defined, can't have port and socket for the same listener.
Apparently prior to 2.2.2 this double listening method definition was not leading to a blocking error.