Cannot upload certificate because it cannot be bundled using Cloadflare trust store

When replacing a SSL certificate and key in the SSL/TLS section I get the following error:

“The certificate chain you uploaded cannot be bundled using Cloudflare’s trust store. Please check your input and try again. (Code: 2100)”

Why is this? How do I troubleshoot this?

Additional information: I checked the certificate and chain PEM and this is correct according to it is a valid chain.

Cloudflare support responded to me (in a private ticket) that most of the times this is caused by a mismatch in certificate and private key. In my case this wasn’t the case.

I turned out that one of the root certificates was not thrusted by Cloudflare.

I figured this out with the help of a tip from Cloudflare support: When you select “Bundle method” as “User Defined”, the certificate will be uploaded without error (at least if the certificate and key matches and are in the correct format). I then saw an SSL error in the browser of my website. When inspecting the certificate in the browser I saw that one of the root certificates was unthrusted.

Hope this helps others running into this issue.

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