Hi, connecting from macOS and Windows to the Ubuntu server are ok, but connecting from Ubuntu is not. Please send us the client side logs on after having reproduced the problem to forum[at]nomachine[dot]com.
To gather logs, please follow the instructions here https://kb.nomachine.com/DT07S00244#2.
After which try disabling hardware decoding on Ubuntu following the instructions here: https://kb.nomachine.com/AR07U01202. You should set the key to <option key=”Enable hardware accelerated decoding” value=”disabled” /> in the player.cfg file.