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.tld
• www.domainALT.tld
• domainALT.tld/pfad/
• http://domainALT.tld
• http://www.domainALT.tld
• http://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.tld
• https://www.domainALT.tld
• https://www.domainALT.tld/pfad/

Workaround via an additional tenant

To enable secure HTTPS redirection (https://domainALT.tld to https://www.domainNEU.tld) without causing a security warning, you can assign the old domain to a separate tenant and set up the redirect there with a valid SSL certificate.

1. Book tenant: Book another tenant if none exists yet.
2. Assign new domain: Assign the domain domainNEU.tld to the new tenant and temporarily save it as its main domain.
3. Place SSL order: Place the SSL order for domainNEU.tld on this tenant to generate the certificate.
4. Create transition domain: Create a new subdomain (e.g. temp.domainNEU.tld) and temporarily assign it as the main domain. This releases domainNEU.tld again.
5. Transfer new domain to main system: Assign the domain domainNEU.tld to your main system and activate it there as the main domain.
6. Transfer old domain to tenant: Assign the domain domainALT.tld to the new tenant and activate it as its main domain.
7. Set up 301 redirect: Set up the following 301 redirect on this new tenant: ^/*;https://www.domainNEU.tld/*;301;L

Alternative workaround via external domain provider

If possible, you can also handle the redirect and SSL certificate through your external domain provider.

1. Add domain at provider: Add domainALT.tld at your external domain provider.
2. Order SSL certificate: Order an SSL certificate for domainALT.tld there.
3. Set up redirect: Set up the redirect from domainALT.tld to domainNEU.tld directly at the external provider.