In my relatively short experience, sometimes NX sessions get broken and you get a blank screen. For us that may be the NX server is hosed, or the X server, and we just restart them (that may sound odd but we usually only have one or two users on each “server”). That may not be so palatable for your system with many users working fine.
When your user is NOT connected, do you see any evidence that they still have an active/disconnected session?
e.g. any processes in ‘ps -ef’
and/or any sessions in /usr/NX/bin/nxserver –history