Not rewriting CDN URLs when using SSL

I’m encountering a strange problem when using WP Super Cache in conjunction with MaxCDN and SSL.

My entire site is accessible only via https (http gets redirected). Also I have MaxCDN configured to use SSL so that my entire site is secured and I have no mixed content errors.

The problem is, when I access my site via https, the URLs of my static content no longer gets rewritten.

I.E. my site is https://www.discountmonster.co.uk, and my SSL CDN is https://discountpull-titanwebsolution.netdna-ssl.com. When accessing via https, the images are being served from discountmonster.co.uk, not the CDN. If I disable SSL and access the site via http, it rewrites the URLs correctly and serves them from the CDN.

Any ideas?

https://www.ads-software.com/plugins/wp-super-cache/