I'm very familiar with Flatsome theme and in fact use it on one of my clients website and I never had such a problem. I think the issue comes from a fact that you host a single site and it has something to do with default server configuration located in /etc/nginx/sites-enabled/default
Can you check if you have your domain name listed under both /etc/nginx/sites-available and /etc/nginx/sites-enabled ?
And, please post the content of the following command:
cat /etc/nginx/sites-enabled/default