I facing to the same problem after I changed "Search Engine Friendly URLs" to "NO" in System Global Configuration.But once I changed back to "Yes" it work properly. But I am not willing to use this. Eventhough I changed this config to not but when I click the "Login" URL still shown "index.php/component/users/?view=login&tmpl=blankpage"
I check from module database found config is "index.php?option=com_users&view=login"
I unpublished this module but Login module still show at its place and not working.