If you're using cursors, then you've probably closed the cursor. Check out MSDN for more information (3)

If you're not using cursors, then it could be an issue with the server, check the log.

And before it's lost to the sands of time, the KB article:

Article ID: 151693 - Last Review: October 3, 2003 - Revision: 3.0

FIX: Err Msg "Unknown Token Received from SQL Server"

Symptoms

When you set the statement options to use a server-side cursor and prepare a select statement on a SQL Server system table, the first execution of the select creates the cursor successfully. After you close this cursor, if you execute the prepared statement again, the following error message appears:

unknown token received from SQL Server.