Asset URLs not HTTPS
Yii will check for the following server environment variables when determining if it’s an SSL request:


HTTPS – should be set to “on”, “ON”, or “1”; or
HTTP_X_FORWARDED_PROTO – should be set to “https” or “HTTPS”


Apparently this is a common issue when running PHP on Nginx, since that HTTPS env variable is only an Apache convention.

Here’s an article that demonstrates how to fix it. According to that, you just need to add the following to your server config.

fastcgi_param HTTPS on;


You can probably add it right before/after your other fastcgi_param line.