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That sound similar to a recent issues with a 403 that was throwing the error. The team is aware and working on that.
(You can test that yourself if you flatten/inline all the documents for the custom error page into a single file (including images), we use these) :
- GitHub - cloudflare/collapsify: Collapsify inlines all the resources of a page into a single document
- collapsify - npm
If related, you’ll see
Response code 403 (Forbidden)