A domain web-server alias (not the CNAME alias) is shown as default Plesk page:
Plesk DNS server is not authoritative for the web-server domain alias:
# dig SOA alias.example.com | grep -A1 “AUTHORITY SECTION”
;; AUTHORITY SECTION:
example.com 86400 IN NS somedns.local.
IP address of web-server alias for domain differs from IP address for the main domain:
# dig +short alias.example.com A
# dig +short example.com A
Incorrect DNS configuration. IP address of web server alias for domain must match the IP address of the main domain.
Access authoritative DNS server and change the A records for a domain alias so they will match the main domain’s A records.