Some firewalls will block the Calendar from loading. If Calendar isn't loading - disable your firewall completely and reload the Calendar page. If this makes Calendar load, you will have to whitelist (unblock / allow) Cloudbeds Calendar IP addresses in your firewall. This way you can still have your firewall on and access the Calendar.
Search your firewall manuals on how to whitelist or unblock specific IP addresses.
IP addresses to whitelist: