This problem has been evidenced with a Small Business Cloud Server (SBCS) which allows up to ten connections.
When an active session created on a node is closed/terminated upon a restart of the server (e.g. 'nxserver --restart'), not all data is cleaned and such session is still counted in the SBCS connections limit.

As a consequence, when connecting to any of the SBCS nodes, the following error is raised even if the connection limit is not reached:
"The session negotiation failed. Error: reached the maximum number of concurrent connections allowed by the Cloud Server".

Workaround is to execute the following command on SBCS. Run in a terminal:
$ sudo nxserver --history
or:
$ sudo nxserver --restart