WordPress Admin Broken with Cloudflare Enabled - Can't Save Posts or Preview Themes

What is the name of the domain?

What is the issue you’re encountering

When Cloudflare proxy (orange cloud) is enabled:1 Cannot save WordPress posts - Editor shows “Offline” error. 2 Theme customizer/preview fails - Stuck loading or blank. No errors in browser console - Only generic “Offline” message.

What steps have you taken to resolve the issue?

Disabled all Cloudflare security features (Free plan, no WAF).
Created Page Rule: domain.com/wp-admin → Cache Level: Bypass.
Paused Cloudflare → Issues disappear (confirming CF is the cause).