This is reported on RBS Servers, usually during installation, when there is already another server running on the same computer and using the RBS Server's port number.

If the RBS Server attempts to use port 21, and your computer already runs an FTP server, this error may happen.

If your computer is already running an RBS Server as a Service, and you try to run another instanace of the RBS Server as an application, this error may happen.

This problem is basically a Port Conflict, and is easy to fix.

Find out which other Servers are running on your computer and either stop them or change the port number of your RBS Server to a different port. The default port number for RBS Servers is 2774.

You can also change your server's port number on the Properties screen of RBS Manager. Doing so will require manually logging in and changing each Endpoint's port number in the endpoints admin interface.