I am puzzling as to why Facebook reports this when I use Facebook's debugger and when I try to add a site running on an ISPConfig3 server to a Facebook business page. " could not resolve the host name into a valid IP address". For what it is worth, the sites all resolve and work as expected. Some of the sites have dns on different servers and some are lazily set up with both dns pointing to the ispconfig3 server. For example, Greenlink.org has different dns servers, and Hcfa.com has both name servers pointing to the ispconfig3 server. No redirects other than to SSL. Debian 10, but the sites are still using php 7.x. All the sites on the server appear to give same results. Any thoughts?