A customer added the domain domain.tld into their account in PA Control Panel, created Apache website on the domain and added wildcard subdomain *.domain.tld to the website. Later, customer removed the domain completely from their account and re-added it anew.
Upon adding the same wildcard subdomain to the domain domain.tld the error is displayed:
Virtual host '*.domain.tld' already exist.
POA did not remove wildcard subdomain when customer removed parent domain domain.tld.
The problem is fixed in POA 5.4.9, upgrade the installation to this or later version. See POA 5.4.9 Release Notes.