The server uses the /usr/NX/var/db as a backend for handling sessions.  In some cases users reported that the /usr/NX/var/db/failed directory is filled up rapidly and can contain gigabytes of data.

Clearing the session history recovers needed space in the affected filesystem.

You can reset the history backlog by running:

$ sudo /usr/NX/bin/nxserver --history clear

Please note that this will not clear sessions currently connected.