I was getting the 404 error on about 50% of my sites it was driving me crazy because it worked on some site and others it gave the 404 error! My persistence payed off!
I didn’t have to uninstall the plugin to do this but I did deactivate it before I did it.
Under Common Settings Change to:
Reactivate the plugin and it should work.
I have not tested any of the other settings but the one I used that it didn’t work on was [Custom Structure.]
I would stay logged in until you are sure it works You can click the link
[Your login page is now here: [https://yourwebsite/l9w8ilewr4y7/]. Bookmark this page!] Click right and open in a new window.
If you are redirected to admin area it worked if you still see the 404 you can still disable the plugin so you don’t get locked out.
Protecting the /xmlrpc.php page as well.
WordPress has a file known as xmlrpc.php that’s useful but has led to some security issues.
I also block the webpage [/xmlrpc.php] with my firewall [Wordfence] I get 100s of bots a day they usually hit the login page 2 times then the /xmlrpc.php page. But be forewarned that some plugins may need that page to work properly. I haven’t had that problem yet but if you do you can put there IP in the whitelist.
Wordfence has a informative page on this issue. You might want toread over that before you block the page.