Answer
R&D's confirmed workaround for this: do the initial fresh installation using a self-signed certificate for whatever proxy combination you're using, and only afterward replace it with your internally-issued CA certificate by running update_http_certificates.sh to update the WebServer certificate. Trying to use the internal-CA certificate directly during the initial installation itself is what triggers the signature-verification failure; installing first with a self-signed cert and swapping it in afterward avoids the issue.