We would recommend setting a Page Rule to exclude the admin section of your CMS from Cloudflare. Please do the following on the Page Rule interface (we'll use WordPress admin section as an example):
1. Go to the Cloudflare Page Rule app.
2. Input the URL for the admin section, which is in this format: yourdomain.com/wp-admin/
3. Place a * before and after the URL, so it reads as *yourdomain.com/wp-admin/*.
4. For "Cache Level", choose "Bypass"
5. For Performance, choose "Disable Performance"
6. For security, choose "Disable Security"
7. Save the rule
It will take 3 minutes for the rule to take effect.
Note: If you are not using Wordpress, substitute the "/wp-admin/*" piece with the appropriate admin and login section for your CMS system.