Thanks alot. I tried this but those 3 URLs were still included as http links instead of https.
In the end it was an easy solution:
fastcgi_param HTTPS $http_x_mb_https;
That variable wasn’t set (not sure where I expected it to be defined). So I manually added a
set $http_x_mb_https on;
and now it works. For some reason I expected that variable to be set somewhere - I shouldn’t have blindly copied everything.