Only Office (error: bad healthcheck status)

I followed the SpaceInvader video for setting up OnlyOffice. However at the very last step when I click save, an error pop out and say "Error when trying to connect (bad healthcheck status". I did tried to remove the cert and key file and recopy again but still not working. I am still a newbie. Could anyone tell me what is the problem?

I have copy and paste my conf file below for referrence as well: -

# only office doc server

server {
    listen 443 ssl;

    server_name onlyoffice.*;

    include /config/nginx/ssl.conf;

    client_max_body_size 0;

    location / {
                include /config/nginx/proxy.conf;
                resolver 127.0.0.11 valid=30s;
                set $upstream_docs OnlyOfficeDocumentServer;
                proxy_pass https://$upstream_docs:443;
                proxy_redirect off;
                proxy_set_header Host $host;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header X-Forwarded-Host $server_name;
                proxy_set_header X-Forwarded-Proto $scheme;
        }
}