503 Error

Hi, my sitemap files gives a 503 error but they work after refresh. This is what I get with curl:

curl -I sitemap.xml
HTTP/1.1 503 Service Unavailable
Content-Type: text/html; charset=utf-8
Retry-After: 15
Content-Length: 464
Accept-Ranges: bytes
Date: Mon, 26 May 2014 14:52:10 GMT
Age: 0
Connection: close
X-Cache: MISS
X-Varnish-User:

I am using nginx and varnish so I guess that my server settings could be causing the issue. I’ve read in the plugin settings that I should add some nginx rules when I get 404 errors, but I am getting 503.

I will apreciate your suggestions.

Thank you.

https://wordpress.org/plugins/google-sitemap-generator/