Login 404

It looks like you might be running a plugin or theme feature that intercepts logins and redirects to a page on your site – this is not default WordPress behaviour. The configuration is seemingly set to a page that does not exist in your installation (with ID of 3911).

Try turning off plugins and using a default twenty* theme to see if you can isolate which plugin is causing the redirect or whether it is your theme. You could also look into your settings pages to see if you can identify a configuration option to set the destination of logins and logouts.