So, if you were presented with the screen for old and new passwords, in a default ZC install as obtained from this site (not necessarily a one-click install), then this represents that the old or temporary password was good/successful. Whatever password was used to get to this screen is the same password that should be used for the old password. The new password should be different than the previous four passwords used and should meet the minimum criteria.

If this still fails such that unable to return to that screen, then would suggest using the "forgotten password" sql explained here: http://www.zen-cart.com/content.php?44
Then once logged in, delete that user profile that is giving a problem and recreate it.