Time out error behind load balancing

Hi there, sorry for my bad english I hope I’ll be understandable

I’m using WordPress for my website Blizzheart.com since the begining and I’ve recently migrate to another server.

Previously I was using an SSL certificate installed directly on my apache server. But since some days I’m using a load balancer for the encryption. It means that my server send informations via port 80 in http to a load balancer, the load balancer encrypt these informations with his own SSL certificate and serve the url with https protocol.

Everything seems to be configured nicely, but I’ve an error provided by the index.php of WordPress.

If index.php of WordPress is the priority file (for example if I delete index.html or if I change priority order in htaccess) ALL URLs based on blizzheart.com domain provides 504 error !

If I put an index.html (As is currently) my website works perfectly (here is an url of my wordpress install) but of course we can’t access to my wordpress home (index.html instead).

Did you have any idea ?

Thanks a lot for your help