Why Default Parallels Page is shown when trying to open Customers Control Panel using IP address or hostname for customer's website, e.g.
This is an expected behavior in PPA. Such login way could be used with Plesk Panel servers, but not in PPA, because IP address/server of management node of PPA could be different from IP address/server of customers domain.
All customers should use hostname or IP address of management node
https://<IP_management node>:8443 to connect to Customers Control Panel.