When I try to save the User-defined Chart, I get an error saying "can not
save this workbook because another open workbook with the same name or
add-in. Choose a different name, or close the other workbook or add-in before
saving."

I don't have an add-in interfering with this, as far as I know. I save the
workbook under another name and the User-defined Chart is not saved.

How can I save a User-defined Chart and how can I make it my default?

I am using MS Excel 2000 (9.0.3821 SR1).