Here is the Headers info from Chrome dev tools, do you see anything amiss?
Request URL:http://www.analytics.mydomain.se/ Request Method:POST Status Code:200 OK Request Headersview source Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Encoding:gzip,deflate,sdch Accept-Language:en-US,en;q=0.8 Cache-Control:max-age=0 Connection:keep-alive Content-Length:114 Content-Type:application/x-www-form-urlencoded DNT:1 Host:www.analytics.mydomain.se Origin:http://www.analytics.mydomain.se Referer:http://www.analytics.mydomain.se/ User-Agent:Mozilla/5.0 (Windows NT 6.2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1500.72 Safari/537.36 Form Dataview sourceview URL encoded form_login:mylogin form_password:XXXXXX form_nonce:TTTTTTTT Response Headersview source Accept-Ranges:bytes age:0 Cache-Control:no-store, must-revalidate, private, no-store, no-cache, must-revalidate, no-transform, max-age=0 Connection:keep-alive Content-Encoding:gzip Content-Length:1649 Content-Length:1649 Content-Type:text/html; charset=utf-8 Date:Tue, 30 Jul 2013 11:01:42 GMT Pragma:no-cache Pragma: Server:- Web acceleration by http://www.unixy.net/varnish Set-Cookie:PIWIK_SESSID=71504ccc576010b36325ed1b9fa2cd; path=/; domain=www.mydomain.se; HttpOnly Vary:User-Agent,Accept-Encoding Via:1.1 varnish X-Cache:MISS X-Cacheable:YES X-Frame-Options:sameorigin X-Varnish:232828836