I added the line [code]
proxy_buffering off;
[/code]
for parkeddomain and problem seems solved.. recommended for performance or stability can have a different setting vhost.conf and nginx.conf thanks.

my vhost.conf

[code]
server {
access_log off;

error_log /var/log/nginx/error.log warn;
listen 80;
server_name maindomain.com www.maindomain.com;

# uncomment location below to make nginx serve static files instead of Apache
# NOTE this will cause issues with bandwidth accounting as files wont be logged
location ~* \.(gif|jpg|jpeg|png|ico|js|css|exe|zip|rar)$ {
root /home/user/public_html;
}

location / {
proxy_send_timeout 90;
proxy_read_timeout 90;

proxy_buffer_size 4k;
# you can increase proxy_buffers here to suppress "an upstream response
# is buffered to a temporary file" warning
proxy_buffers 16 32k;
proxy_busy_buffers_size 64k;
proxy_temp_file_write_size 64k;

proxy_connect_timeout 30s;

proxy_redirect http://www.maindomain.com:81 http://www.maindomain.com;
proxy_redirect http://maindomain.com:81 http://maindomain.com;

proxy_pass http://serverip:81/;

proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
}
server {
access_log off;

error_log /var/log/nginx/error.log warn;
listen 80;
server_name forum.maindomain.com www.forum.maindomain.com;

# uncomment location below to make nginx serve static files instead of Apache
# NOTE this will cause issues with bandwidth accounting as files wont be logged
location ~* \.(gif|jpg|jpeg|ico|png|js|css)$ {
root /home/user/public_html/community;
}

location / {
proxy_send_timeout 90;
proxy_read_timeout 90;

proxy_buffer_size 4k;
# you can increase proxy_buffers here to suppress "an upstream response
# is buffered to a temporary file" warning
proxy_buffers 16 32k;
proxy_busy_buffers_size 64k;
proxy_temp_file_write_size 64k;

proxy_connect_timeout 30s;

proxy_redirect http://www.forum.maindomain.com:81 http://www.forum.maindomain.com;
proxy_redirect http://forum.maindomain.com:81 http://forum.maindomain.com;

proxy_pass http://serverip:81/;

proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
}
server {
access_log off;

error_log /var/log/nginx/error.log warn;
listen 80;
server_name domainparked.com www.domainparked.com;

# uncomment location below to make nginx serve static files instead of Apache
# NOTE this will cause issues with bandwidth accounting as files wont be logged
#location ~* \.(gif|jpg|jpeg|png|ico|js|css|exe|zip|rar)$ {
#root /home/user/public_html;
#}

location / {
proxy_send_timeout 90;
proxy_read_timeout 90;

proxy_buffering off; #add this line
#proxy_buffer_size 4k;
# you can increase proxy_buffers here to suppress "an upstream response
# is buffered to a temporary file" warning
#proxy_buffers 16 32k;
#proxy_busy_buffers_size 64k;
#proxy_temp_file_write_size 64k;

proxy_connect_timeout 30s;

proxy_redirect http://www.domainparked.com:81 http://www.domainparked.com;
proxy_redirect http://domainparked.com:81 http://domainparked.com;

proxy_pass http://serverip:81/;

proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
}
[/code]