Username validation always fails for Drupal users who don't have permission to change it.

The validation of usernames added by this module is applied even if no username appears in the user's account form.

This line in magento_users_user():

if (count(explode(' ', $edit['name'])) !=2) {

should read:

if (isset($edit['name']) and count(explode(' ', $edit['name'])) !=2) {

so that the username isn't validated if it isn't present in the form data.