The crucial hint is this: “One of the changes of the latest systemd update is that it respects the “ro” boot option now and does indeed mount the root partition read-only if that option is present. Before it was just ignored.”
dmesg did indeed show that the system boots with ‘ro’.
To fix this, I edited the boot loader command script (hit ‘e’ while the Grub boot menu shows, remove the ‘ro’ then reboot with F10). Have yet to figure out how to solve this permanently, but I could subsequently install the latest kernel update and apply the NoMachine 5.00.47 update.