It's possible to connect to the server, however, after having logged in the user gets white screen for about 10-15 seconds.

A dialogue box with the following error message appears on the server:

    Unable to contact settings server
    Failed to connect to socket /tmp/dbus-xxxxxxxxxx: Connection refused.

    
This issue is caused by a system update.

As a workaround NoMachine development team suggests to uncomment the DefaultDesktopCommand node.cfg key and edit as follows:
DefaultDesktopCommand "/usr/bin/dbus-launch /usr/bin/startxfce4".