When the physical display session fails for some reasons (e.g. an Xorg problem), the nxserver --daemon process, under very specific conditions, suddenly terminates while handling such failure.

As a consequence, 'nxserver --status' reports that the nxserver service is disabled.

As a temporary workaround, start the service:

sudo /etc/NX/nxserver --startup

When this problem occurs, server logs report:

2018-09-27 18:07:55 666.870  2817 NXSERVER WARNING! Session <session ID> crashed on display 0.
2018-09-27 18:12:18 381.708  2817 NXSERVER ERROR! <346EEF89> Exception:
2018-09-27 18:12:18 381.818  2817 NXSERVER ERROR! <346EEF89> Can't call method "isSetToRemove" on an undefined value at /usr/NX/lib/perl/nxserver/NXLocalSession.pm line 1237.n