Looking at the webervers error logs, i´ve found a lot of entries in one of them in an ISPConfig 3 environment:
[crit] 26595#0: *28425 open() "/var/www/site/web/" failed (13: Permission denied), client: xx.xx.xx.xx, server: host.tld, request: "GET / HTTP/1.1", host: "www.host.tld"
I spent some minutes to figure it out. In my case it was a permission error on website root folder (permissions were set to 711).
A simple command “chmod 751 web” fixes my problem.