Error 500 while accessing a php file

Hello,

I am getting error 500 while accessing a php file.

I am using CentOS 7.4 with Webin 1.870, virtualmin 6.02 and usermin 1,730. I'm using a Digital Ocean droplet with an extra 100gb disk mounted in /home (folder permission is 711).

Virtualmin Apache Error log shows: [Sat Dec 23 22:52:07.730149 2017] [cgi:error] [pid 8217] [client my_ip:53944] End of script output before headers: php7.1.cgi

Steps I've taken to get this result:

I mounted my server, I created a virtual server using the main server domain (the same one I use to connect to port 10000), I created the SSL certificate, I installed PHP versions 5.6 and 7.1, I selected version 7.1 to use on that virtual server and put an index.php in the public_html folder of it, but I'm getting 500 error while accessing the site. I have no idea why :/