Hi Aubert887,
Yes, it is possible to retrieve the sessions lost in the master node on the secondary node, but NoMachine has to be configured to not run sessions on the Enterprise Terminal Server host, but only on the Terminal Server Node(s).
I.e. only sessions running on the nodes can be recovered, if the server goes down.
You can find in the following guide more details and a step by step example to configure a HA multinode setup: https://www.nomachine.com/DT06O00142&dn=failover#3.4 (check the section 3.4. Setting-up Two Enterprise Terminal Servers in a Failover Cluster)