@Krassus,
1. Make sure the ports (443 and 7444) are open and connectable between vCenter/Hosts and Skyline on the network.
2. If there is a firewall make sure there is no SSL filtering for Skyline
3. You could try to connect with vCenter using the curl command below, Open the console of Skyline VM
curl -v https://vcenterfqdn:443
curl -v https://vcenterfqdn:7444
4. Lastly, You may try to delete the vCenter from Skyline Collector and re-add afresh.
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Regards,
Shen