Many thanks Britgirl, resetting permissions did the trick. However, while I am now able to connect to my Mac while its already on with me logged in, I typically run it as a headless server (i.e. no keyboard, mouse or display monitor) and this presents a new problem which i have not had before under the old version of NoMachine.
When I try to connect from my PC, i am now presented with a message “Waiting for the desktop user to authorize your connection.” Authorization will of course never happens because the Mac is headless. Is there a workaround for this?
Thanks again for your excellent help!