After Nginx reverse proxy has been disabled, a domain starts to produce the Web Server's Default Page. However, if Nginx is enabled again, the problem disappears.
This problem may be caused by a mismatch of the IP address on which the domain's virtual host is configured in Plesk and the IP address to which the domain resolves.
Apache maps virtual hosts to IP addresses strictly: a virtual host for a domain is configured on a single IP address and works only when both the IP address from which the HTTP request came and the requested domain name of the HTTP request match those defined in the virtual host configuration. However, when Nginx is enabled, all virtual hosts on the Apache side are assigned to a single IP address (127.0.0.1) and even if an IP/hostname mismatch occurs, it does not cause problems.
Check the IP address in the domain's hosting setup (Subscriptions → subscription → Websites & Domains tab → Web Hosting Access) and compare it with the IP address which is returned by the
nslookup command executed locally (i.e., on a workstation or laptop from which access to the website failed):
If the domain resolves to the IP address that is assigned to the same Plesk server, re-assign the domain to this IP address in the subscription properties using the Change Hosting Settings menu item.
Otherwise, change the IP address in the DNS records of the domain's DNS zone to have the same IP address as the one on which the virtual host is set up on the Plesk side.