probably you have misconfigured nginx conf and you have bad chmod
Try This
chown www-data:www-data -R /var/www/yourforumfolder
And this
listen 80;
server_name yourdomain.com www.yourdomain.com;
# note that these lines are originally from the "location /" block
root /var/www/yourforum;
index index.php index.html index.htm;
location / { try_files $uri $uri/ /index.php?$query_string; }
location /api { try_files $uri $uri/ /api.php?$query_string; }
location /admin { try_files $uri $uri/ /admin.php?$query_string; }
location /flarum {
deny all;
return 404;
}
location ~ \.php$ {
try_files $uri =404;
fastcgi_pass 127.0.0.1:9000;
fastcgi_index index.php;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
include fastcgi_params;
}
location ~* \.html$ {
expires -1;
}
location ~* \.(css|js|gif|jpe?g|png)$ {
expires 1M;
add_header Pragma public;
add_header Cache-Control "public, must-revalidate, proxy-revalidate";
}
gzip on;
gzip_http_version 1.1;
gzip_vary on;
gzip_comp_level 6;
gzip_proxied any;
gzip_types application/atom+xml
application/javascript
application/json
application/vnd.ms-fontobject
application/x-font-ttf
application/x-web-app-manifest+json
application/xhtml+xml
application/xml
font/opentype
image/svg+xml
image/x-icon
text/css
#text/html -- text/html is gzipped by default by nginx
text/plain
text/xml;
gzip_buffers 16 8k;
gzip_disable "MSIE [1-6]\.(?!.*SV1)";
}
remember to restart the server service nginx restart