Login form action remains http for unmapped domain on HTTPS only site

Hi there,

we have the following config:

- Subdomain Multisite
- HTTPS only for original and mapped domains
- FORCE_SSL_ADMIN is set to true in wp-config

Nginx rewrite rules to rewrite all HTTP requests to HTTPS.

On the main site (no Domain mapping) everything works fine. On sites with a mapped domain the login also works fine with the mapped domain.
On a subdomain (virgin site in the multisite) without domain mapping the login forms action remains on http thus no login is possible.

All the other ressources are correctly loaded via HTTPS. Certificate is obviously working fine.

Domain Mapping Plugin config is set to:

- Use mapped domain everywhere
- Force https for everything

I enabled support access for the installation and left the URL to the login there in the note field. So take a look yourself.

Eager to hear from you!

Jan