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