When requested by the Support Team, or on the Forums, it might be necessary to disable H.264 hardware decoding on the client side (the device you are connecting from) and force NoMachine to use software decoding. It can be done by following the instructions below.

First exit the Player by going to the !M icon in the system tray and selecting "Quit NoMachine player".

Edit the player.cfg file in the user's home on the client side.

This file is: 
$HOME/.nx/config/player.cfg on Linux and macOS and %USERPROFILE%\.nx\config\player.cfg on Windows.

Edit the "Enable hardware accelerated decoding" key and set the appropriate value as indicated by the Support Team.

This key accepts 3 values:
1) true  HW decoding is enabled
2) false  HW decoding is disabled, but the codec library for HW decoding is still loaded on the system
3) disabled  HW decoding is completely disabled, the codec library for HW decoding is never loaded and the checkbox is grey out inside UI panel

For example:

<option key="Enable hardware accelerated decoding" value="false" />

or:

<option key="Enable hardware accelerated decoding" value="disabled" />

Note
Options 1) and 2) can be set also via the UI: open the NoMachine menu panel inside the session by Ctrl+Alt+0 or click on the page peel at the top right corner of the window. Then click on Display -> Change settings. In that panel click on the Modify button to open the advanced display settings.  Click on 'Disable client side hardware decoding' to disable it.