You receive the security warning because for subdomains that serve as redirect domains after the change, technically no SSL certificates are delivered anymore, even if one existed before the switch. The redirect from the old domain including https:// therefore fails or leads to the security warning, as the certificate is only issued for the active main domain.The main system only delivers the SSL certificate for the active main domain (domainNEU.tld). Subdomains (domainALT.tld) do redirect to the main domain but no longer receive their own SSL certificate, which causes the security warning at https://domainALT.tld.Working redirect examples (without explicit HTTPS)The following calls correctly redirect to the new main domain:domainALT.tldwww.domainALT.tlddomainALT.tld/pfad/http://domainALT.tldhttp://www.domainALT.tldhttp://domainALT.tld/pfad/Non-working redirect examples (with explicit HTTPS)The following calls show the security warning because no valid SSL certificate is delivered for the old domain:https://domainALT.tldhttps://www...