If a Proxmox Host is behind a Reverse Proxy that responds with HTTP/2, the api will return "invalid HTTP Response". This is because HTTP is parsed manually: https://github.com/CpuID/pve2-api-php-client/blob/b9ef9cb4f040cf71bfa5281b1e31bfae7a984474/pve2_api.class.php#L245