I have the same problem — after updates (both system and local client):
/usr/NX/bin/nxserver –version: NoMachine – Version 5.1.62 (on a RHEL7)
Mac OSX (High Sierra) Client: NoMachine – Version 5.3.25 (5.3.25)
Upon connection: “Could not connect to the server. Error is 61: Connection refused”
while “sudo /usr/NX/bin/nxserver –start” shows:
NX> 161 Enabled service: nxserver.
NX> 162 Disabled service: nxnode.
NX> 162 Disabled service: nxd.
My firewall port 4000 is open… and above all, it was working over the past year.