This issue occurs if you try to assign a name that is not allowed to the port name. For example, a name with alphabetic characters after an index (e.g., "mybus(3:0)one") would cause this problem.
This problem has been fixed in the latest 10.1 Service Pack available at:
http://www.xilinx.com/support/download/The first service pack containing the fix is 10.1 Service Pack 3.