XML asset fails to load using https

In case this helps anyone in the future:

The problem was not a WordPress issue, it was a server configuration issue.

Useful explanation on checking which stream wrappers are enabled on your server (used this to make sure https was enabled on my server) Enable Stream Wrappers

Someone with the same issue found that their certificate failed to verify:
Certificate Verification Fails

The issue was fixed by our hosting provider, with the following explanation:

The problem is caused by the the self signed certificate on the
server. We could either allow the self signed certificate in the php
setup or load a valid SSL certificate.

The issue was fixed by creating a self signed certificate which
matches the Openssl common name. This fixed all libxml errors

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