The hypervisors (ESXi hosts) must allow http traffic over ports 80/443. You can verify and enable this using the vSphere client:
1. Select the hypervisor in the inventory.
2. Click on the Configuration tab.
3. Select ‘Security Profile’ in the Software grouping.
4. Click ‘Properties…’ at the top of the Firewall settings.
5. Verify or enable ‘httpClient’ in the list of firewall settings.
I hope that helps!