error when previewing website

Hello,
We are using the news template in our website www.agronegocios.eu. I was successfull installing the GK Quickstart package and I am able to manage with the template thru the Joomla CMS with no major problems, but when I try to preview the website I get the following error:
502 Bad Gateway nginx
Maybe this is a hosting problem, perhaps anyone already faced the same issue?
My environment: CentOS 6.5; PHP 5.3.3; nginx/1.5.0; MySQL 5.1.73; Joomla 3.2.3
I would appreciate any sugestion! Thanks.

Looks like a server side problem.
Also you might check disabling modules that are available on homepage - perhaps one of them is causing the problem.

I took your sugestion and determined that a tab module in the demo was causing the problem (sports news). After disabling it the error was gone.

Do you have an access to server logs (php error log, apache error log) - perhaps you could find thee some more clues.

Sorry, not apache error log - you need nginx error log.

Here is the content of nginx error log (just for your knowledge because I got rid of the problem in other way):
2014/04/21 06:05:40 [error] 3807#0: *214 upstream sent invalid header while reading response header from upstream, client: 66.249.66.131, server: agronegocios.eu, request: "GET / HTTP/1.1", upstream: "http://176.111.109.123:7080/", host: "agronegocios.eu"
2014/04/21 10:10:23 [error] 3807#0: *225 upstream sent invalid header while reading response header from upstream, client: 68.180.224.177, server: agronegocios.eu, request: "GET / HTTP/1.1", upstream: "http://176.111.109.123:7080/", host: "agronegocios.eu"
2014/04/21 10:30:39 [error] 3807#0: *228 upstream sent invalid header while reading response header from upstream, client: 95.136.96.5, server: agronegocios.eu, request: "GET / HTTP/1.1", upstream: "http://176.111.109.123:7080/", host: "agronegocios.eu"

For sure those are not connected. Please enable error reporting in joomla and enable module again - it should show the cause of the problem.