All domains on a Plesk server are showing Plesk login page when option The specified domain or subdomain that resolves to the server IP address but is not used for hosting is enabled in Tools & Settings > Customize Plesk URL
This is Plesk bug ID #PPPM-12289 which is going to be fixed in future Plesk updates.
Until the bug is fixed, use the following workaround: disable the customized Plesk URL so that Plesk GUI is only accessible via port 8443:
Go to Tools & Settings > Customize Plesk URL > Select the option No custom URLs: Only https://<server-IP-or-hostname>:8443 >
Click OK to apply the changes