Only one instance of Tally.ERP 9 prior to Release 5.0 can be accessed using the gateway.
This error occurs when more than one instances of Tally.ERP 9 Release 4.93 (or) below is accessing single user (silver) license from Tally Gateway Server. For example, suppose Tally.ERP 9 Release 5.0 is running with single user license (configured to Tally Gateway Server with the port number 9999), and more than one instance of Tally.ERP 9 Release 4.93 try to access same license by configuring to 9999 port. In this case the second instance will get the above error.
Only one instance of Tally.ERP 9 Release 4.93 or below will be able to access single user license from Tally Gateway Server.