Logging can be enabled for the server by enabling this key in the /usr/NX/etc/node.cfg and server.cfg files:
SessionLogLevel 7
However problems with network outage are more likely to be identified by looking in the .nx directories in the user’s home on client and server side (more info in https://www.nomachine.com/AR07K00677 on how retrieving logs). You may notice logs similar to “WARNING! No data received […] since […] ” or any other actual error. Log level cannot be changed there, but default should be enough.