HTTPS is downloading file, HTTP aint

Hi all,

I'm pretty new with NGINX but i would like to learn a lot about it. Currently I've installed NGINX on CentOS7 with PHP-FPM. I've installed Let's Encrypt and would like to use HTTPS. The only problem I have is that when you go to the HTTPS site the page will be downloaded, but it works fine at http.

Is there someone that can help me with this?

Thanks in advance!

Koen