This problem happens when for some reasons the NoMachine Connction monitor process terminates without a clean procedure.

The new Connection Monitor process still tries to use previous cookie without cleaning up the db information.

The Connection Monitor log reports:
ServerListener/ServerListener: Wrong cookie '4130E2E8D0AD47A00B578E5B21E6CBA9' provided.

As a consequence, request for generating token to access the Cloud Server node is denied and users are unable to connect to the node:
NXSERVER WARNING! Cannot get one-time password. Cannot connect to the remote server.

As a temporary workaround, clean-up the db by executing in a terminal on the Cloud Server host:

sudo /etc/NX/nxserver --list