Hi and welcome to the forum! Maybe your WordPress installation is corrupted:
You might only need to upload the wp-login.php file back though and not follow the guide entirely, stopping at the section where it tells you to upload all the files from the fresh installation of WordPress, as the rest seems to work fine (even the redirect from wp-admin if it wasn’t for the 404 page). If it still doesn’t work, maybe you might have used a plugin to change your login URL, then disabled it and so it might have forgot the default setting. In the latter case I don’t know which plugin you used to change login URL if you used one, so can’t help you further.