Acronis Cyber Protect, Acronis Cyber Backup: Agent registration fails with "503 service unavailable" if if http_proxy environment variable exists

Symptoms

1. You want to install an agent in an environment with a proxy.
2. You have configured proxy according to the user guide (Acronis Cyber Cloud, Acronis Cyber Protect 15) 
3. Agent registration fails with the following error:

   503 service unavailable