Hi again,
just wanted to clear this up, maybe it's helpful for others: after following https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Installation/ApacheHTTPD/ , I can confirm now that this is fixed by adding
RequestHeader set Forwarded "proto=https"
to our httpd config. Without this line, the attachment links have http:// at the front.
The issue can probably remain closed, thanks for looking into it.
