New node cannot join existing cluster: IOException[Connection Refused]

some more stats on the vm's

*All are running CentOS Linux release 7.5.1804 (Core)
*All run Java 1.8.0_40
*Version of Elasticsearch: 6.2.3
*4cores, 16 GB or RAM, 8GB assigned to elasticsearch via jvm.options

The annoying thing is I'm tailing all relevant logs when trying to join, but nothing appears of this event in the logs of the master node, so we're kind of flying blind there...

Cheers

Jan