Hello gabriele,
To solve the problem you should remove the problematic file and execute
nxserver --list
It will fix the database.
Before that please check the current permissions to that file:
ls -l /usr/NX/var/db/limits/sessions.vdesktop
and gather the logs:
nxserver --debug --collect
Send them to forum[at]nomachine[dot]com making sure to reference your topic.