If the user is not able to login after a Magento version update, you have most likely overwritten the login.phtml template in your local theme. To fix that problem, open \customer\form\login.phtml in your local theme and add the following code…

If a customer cannot log in after a Magento version update, you may have overridden login.phtml in your theme. Open \customer\form\login.phtml in your theme and insert the following code immediately after the form tags.