The default value for the number of permitted concurrent sessions for the Web Service data feed is one (1). More than one active session at a time will result in this error.
Using the Close Data Feed call will usually resolve this issue. Be sure to append the unique session token to the end of this call:
If you do not have the session token any longer, you may need to wait up to 15 minutes before the session is automatically cleared.
An open Web Service session will not time out unless no activity occurs against a session in the time governed by the
MaxRequestInterval constraint (default of 15 minutes).
For more information, see Data Feed API in the documentation.