After changing Site url to https, can’t access login page

I had the same problem.

You need to install the plugin CloudFlare Flexible SSL https://wordpress.org/plugins/cloudflare-flexible-ssl/

As you can’t login in to dashboard so you can’t add that plugin now. But there is a way for this.

Just login to cpanel or just use ftp. Then createa folder inside wp-content named mu-plugins .

Now download that plugin and upload all files of that plugin in mu-plugins folder. Make sure there is no folder inside mu-plugins folder. Just upload the plugin files to mu-plugins. Then this plugin will be active automatically and your problem will be solved right away.

Hope this helps