Site not responsive

I’ve switched today from Suffusion to Nirvana but had to realize that it is not responsive at all. On Chrome/Mac it looks perfectly fine, but on mobile there’s a non-responsive header + title and no articles or body text at all, regardless of the page I select. It also does not look correct if I make the browser window smaller on Mac.

Of course I checked an unchecked the responsive-checkbox and fiddled with other suspicious settings, but to no avail.

Could you please check out my site and tell me what I did wrong?

Thank you very much,

Michael

Website: www.mongolei.or.at