I saw that there was a similar thread on the forum a few months ago, and it was unclear if it was ever resolved. I'm new to proxmox and would like to evaluate it but it's not very confidence-inspiring to have such infrastructure problems with bad certificates. I've managed to manually download the gpg file at http://download.proxmox.com/debian/proxmox-ve-release-6.x.gpg and the sha512sum does match, and I can perform apt-key add, but when I then attempt either apt update or apt-get update, the process errors due to certificate verification failure (could not handshake 144.217.255.162:443). The gpg file is dated 23-may-2019.