Hmmm.. Well, that error is generated when either the password variable OR the user id variable is empty when passed into the changepw function.
Have you modified any files inside your clean theme? If so, double check your work to be sure you preserved any variable names in the forms. If you still have problems, try reinstalling a FRESH copy of the clean theme. If you STILL have problems, rule out themes altogether by installing the professional theme and try to change the password again.
~Brian