A green screen is displayed when the user connects by NoMachine client for Windows v. 6.10.12 to a Linux NoMachine server.

This problem occurs in two different cases:

1) when multiple monitors are connected to the NoMachine server host.  

2) when monitor resolution on server side is higher than FullHD (e.g. one single Ultra HD monitor or multiple monitors with a global resolution higher than 1920x1080). It happens also when monitors are not physically connected to the server host.

This second problem is triggered by GPU/drivers issues with hardware decoding on higher resolution screens.
Upgrading GPU Intel drivers solves this problem.

As an alternative, it's possible to apply the following workaround which is valid for both problems 1) and 2).

Disable the client side hardware decoding:

- Connect and open the !M menu menu inside the session by ctrl+alt+0 
   or click on the page peel in the top right corner of the window.
- Access the Display -> Change settings panel
- Check the 'Disable client side hardware decoding' option

Note:
Both problems 1) and 2) were not visible with previous versions of NoMachine client because the hardware decoding was used by default only for screen resolutions up to 1920x1080. The implementation of this Feature Request: https://www.nomachine.com/FR04R03970 enables HW decoding also for higher resolutions than Full HD.