This has been resolved. It was not a problem with NoMachine, but rather with the KDE Plasma runtime. The LD_LIBRARY_PATH environment variable had been modified in the .bashrc script, so as to point to a slightly different version of QT5, which was incompatible with Plasma. As a result, when the nxserver daemon tried to instantiate a new X-win environment, the request failed. Took me a while to figure it out because the client-side error message had no indication that it was a server-side, Plasma problem.