Certbot 0.27 was written against a draft version of the ACME protocol. There was a late breaking change before the protocol was finalized, which disallowed an empty POST to query account registration.
Some package maintainers backported this change into their python-acme packages, some didn't. It looks like in your case, py27-acme-0.27.1,1 doesn't contain the required fix.
One more question: Since creating NEW certificates works correctly but EXPANDING existing ones does not: How can I erase existing ones, so that I simply can CREATE new certificates including the additional subdomain?
It is a question of time! I also have to update the OS and I don't have time for that until next week. But I currently need a certificate that also covers another subdomain. As a workaround I can only create this one separately and would have to make an individual Virtual Host entry in Apache conf. I would have liked to avoid this.
Creating a new certificate by specifying all the domain names (old and new) will produce the same certificate in the end as attempting to expand a certificate, so if the new route works, use that.
I actually counsel people against using --expand in favor of --cert-namename, which is so much more versatile.