This error pertains to older versions of the C-I Development product. It does not pertain to version 6.
Commercial/Industrial Development Version 5, build 1.13 and earlier required that iteration be turned on, while starting in build 1.14, iteration should be turned off. If you open a workbook with iteration turned off, and then proceed to open a second workbook that requires iteration, you may see a circular reference error. In this case, you should click Cancel to continue leaving the formula as it is. Then go to Tools > Options > Calculation and make sure that Iteration is checked (turned on).