HAPROXY and CloudFront

[carlo_gra]

I've succesfully settled HAProxy on OPNSENS with one frotend and few rules to correctly forward traffic on backend pools (I have two for two different services).

Everything is working properly except cloudfront which is used as CDN for a wordpress website (just for images, css and js).

The main domain points directly to haproxy, the cdn subdomain points to cloudfront.
The mail domain is also settled as "origin" for cloudfront.

When I try to reach cdn.XXXX.TLD I get "502 Bad Gateway".
EDIT: I did realize that also from GTMetrix I get the same error while on the others (pingdom, my browser, google page speed) everything seems  ok.

The frontend uses SSL Offloading, I also used SSL with both wildcard and specific domain.

Any suggestion?