Thank you. THis is not a headless Linux machine.
I was able to solve the problem by logging in locally on the machine and have a complete removal of the package, deleting all the local files (in /usr/NX and in .nx in the home). I then reinstalled the package and it worked.
Previously I connected only remotely and did not delete the local files. Unsure of which one did make the difference, I report them in case others have the same issue.