(bloody)workaround:
Start VNC screensharing via terminal and use a VNC-Client to connect.
Stop NoMachine in Finder menu completely.
Start the app and within the app, start the server again.
-> NoMachine will work again. At least for me. To check back, I broke down my connection without logout/closing the nomachine-window. The issue was up again after re-establing my connection. I got the white screen..
I do not understand, why this is helping, but a restart of nxserver/nxd on commandline do not help…