One hostname failing.
THE CHECKS
Does the subdomain exist in cPanel? Does a DNS record exist for it, at whichever provider holds your DNS? Does it resolve to the correct address? Does it have certificate coverage?
THE COMMON CAUSE WITH EXTERNAL DNS
The subdomain created in cPanel, and no DNS record added at the external provider.
cPanel shows it as created; DNS knows nothing about it.
THE COMMON CAUSE WITH OUR DNS
A typographical error in the name, or the record pointing at the wrong address.
CERTIFICATE COVERAGE
A subdomain resolving but showing a certificate warning needs AutoSSL run, ticking that name.
DOCUMENT ROOT
A subdomain resolving and showing the wrong content usually has the wrong document root.
cPanel > Domains shows the path for each.
WILDCARD RECORDS
If a wildcard exists, subdomains resolve whether you created them or not.
That can mask a missing specific record while producing unexpected content.
CHECKING
Look up the subdomain. Compare the result against your server's address.
