MacOS installation fails, possibly due to proxy

It isn't blocked ports: I am able to sync my vaults with my accounts without issues.

The issue - and I've seen that many times before elsewhere, especially with certain inspection firewalls and proxies is that they choke when trying packet level inspection for an entire download stream across SSL traffic. If the company is a cheapskate they won't sufficiently provision the units and they simply can't keep up. Won't name company or even industry, but last time I saw it this bad was incidentally in the same industry. Go figure.