When I changed the URI to access to my dashboard, when I logged in again, there was a 404 error on the admin dashboard. It seems is something related to permissions and can be easily solved executing the following commands:

find . -type d -exec chmod 755 {} ;
find . -type f -exec chmod 644 {} ;
chmod o+w var media -R

If this doesn’t work, maybe the login form is not redirecting correctly. In that case we need to change the action url in the app/design/adminhtml/default/default/template/login.phtml file. Look for the «form» start tag:

<form method="post" action="" id="loginForm">

Change it to:

<form method="post" action="<?php echo $this->getUrl('adminhtml') ?>" id="loginForm">

that’s it, you have you familiar dashboard back 😀