I am facing the same issue connecting to a Windows 7 machine from a Windows 7 machine both running NoMachine v5.1.26_1, downloaded yesterday. Three monitors of same dimensions at both ends, in “All” multimonitor mode. This setup works find when I use another remote desktop application to log-in and presumably wake up the display. If I connect directly to the remote machine for the first time with NoMachine when the displays are turned off I get white screens in NoMachine, these quickly disappear when I use the other remote desktop application to log-in. I experienced the same thing when connecting from an Android tablet (most recent version), suggesting the issue is at the server and not the client end.
The only other thing that may be a bit unusual about my set up is that I am running a VirtualBox Red Hat Linux 6 client in seamless mode, which sometimes has minor effects on display.