Drupal 7 in subfolder behind reverse proxy, login not working
As I understood, the #action on the page /user is generated by request_uri, the default action of each form, except for few form that are block (check if the user_login block has the same problem).

I can advise you, to check your PHP config in Report -> Status report and then click on the PHP version an try to find the REQUEST_URI information, this can be different from what you are expecting (I guess /drupal/user).

To fixe it, I see only two solutions, submit an issue (or check if that issue already exists) or the second its to use the hook_form_alter and replace the #action by something like that.

$form['#action'] = url(current_path(), array('external' =&gt; FALSE));