This message may occur with ATS (A-Shell Telnet Server) and indicates that you have too many open sessions originating from the same device, or sharing the same IP address. ATS is somewhat more flexible that other A-Shell versions in allowing multiple sessions to share a single node license if they originate from the same device, but since this could be abused by running many terminals through a single terminal server or other proxy service, there is a limit (typically about the same as the number of licensed nodes.)