I assume you’re trying to connect the player with https url?
Can you reach the CMS address in the web browser on the PC with Xibo player?
It might also depend on what kind of SSL certificate you have, for example for the TSL 1.2 there are some registry changes required Enable TLS 1.2 / Strong Crypto in Windows .net Player
I’d also like to know if it is self-signed certificate or "trusted’/‘proper’ certificate, if it is the latter are the intermediate certificates installed correctly as well?
If there are cert chain problems then the following should help you identify (and fix) the issue
Test your server here https://www.ssllabs.com/ssltest/analyze.html
That should tell you where the issue is, if it’s issue with certificate chain, you can test it here https://whatsmychaincert.com/