ERR_CERT_AUTHORITY_INVALID on Cloudflare
Why it happens on Cloudflare
Cloudflare Origin Certificates are only trusted by Cloudflare, so hitting the origin directly in a browser always shows this error.
The fix on Cloudflare
This is expected. Test the origin with an SSL tool set to ignore trust, or install a publicly-trusted certificate on the origin if it must be reachable directly.
Is ERR_CERT_AUTHORITY_INVALID happening on your domain?
An external SSL check builds the chain from scratch with no local trust store shortcuts, so it shows a missing intermediate that your own browser may be silently caching around.
Free, no signup. Runs from outside your network, which is the only way to tell a server fault from a local one.
If that didn't fix it
Cloudflare has its own failure mode, but ERR_CERT_AUTHORITY_INVALID has a wider set of causes that apply everywhere. The most common one overall is: the server serves only the leaf certificate and omits the intermediate bundle.
All causes and fixes for ERR_CERT_AUTHORITY_INVALID