Well the obvious troubleshooting steps you need to do now are to deactivate any plugin that has anything to do with your Login page. Then find out why Cloudflare is caching your login page or whatever else it is doing loading on your login page - very strange unless it is supposed to be there. Then figure out why you have a hidden login form and then figure out what all the other crap is that is loading in the Source Code of your login page.
Logically what is happening is the login post and redirect to your login page probably does not know what the hell is going on since your login page is a total mess.