The system is already opened by another client using a different network interface or database directory. (Subsystem: NS, #149) is seen when you attempt to open the same database with another LNS application but with a different network interface.
For instance, the LNS Object Browser is a low-level utility for opening and manipulating LNS databases. If you have a database open in LonMaker or Izot CT but then try to open the same database with the LNS Object Browser with a different network interface option (including engineered mode), you will see this error. Engineered mode means no network interface which to LNS is a different network interface.
When using the LNS Object Browser, you can use the "Don't Change" option for the network interface, it will then automatically use the current network interface for the network.