Problem is fixed by adopting a proper procedure that requires to upgrade before the Enterprise Terminal Server and then the nodes. The related article has been updated accordingly: https://kb.nomachine.com/AR11T01180

According to the upgrade procedure for a multinode environment (AR11T01180), administrators are recommended to upgrade nodes (also gradually) and then the Enterprise Terminal Server ETS. 

Due to to some node changes from Terminal Server (v7) to Enterprise Terminal Server (v8), the ETS v7 is not able to recognize them. It's then necessary to upgrade also ETS.

As a workaround to upgrade the environment gradually, upgrade the ETS before and then the nodes.