Updated ProxCert...

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The use of Intermediate CA may have grown since Proxo was last built. I wonder if doing the same process with the Intermediate CA as you did with the Root CA could fix your issues. Append the PEM file created from the Intermediate onto the back of the certs.pem that you already have.

Technically, I think each Intermediate is chained either to another Intermediate or to a trusted Root CA. If chained to another Intermediate then eventually that chain must find its way to a trusted Root. I think I remember reading somewhere there was somewhat recent OpenSSL changes on how that trust determination is invoked, maybe those changes were after Proxo.

What versions of LibEay32 and SslEay32 are you using in Proxo? That may have a bearing on things.

Hopefully someone else will have some better ideas for you.