I have great news! I was able to fix the issue on a per-site basis, although going forward it would be good to know how it can be fixed sitewide.
I added the following line to the bottom of my \site-dir\conf\php\php.ini.hbs file and restarted the site:
openssl.cafile=C:\Program Files\Common Files\SSL\cert.pem
Obviously, this is based on the cert.pem (previously cacert.pem file you suggested) in the directory.
Based on this are you able to tell me how I can add that line to some kind of global PHP Ini file for my Local sites?
Thank you.