A network subsite inaccessible after 3.5 upgrade

Hi,

This is probably a tricky one for you to respond to, but I will try anyway. After upgrading our multisite install yesterday at http://mediajungle.dk, I had a call from a user in distress this morning. His subsite http://pesticider.mediajungle.dk doesn't show up in the browser (the title loads though). Attemps to access the site's admin leads to an error page

Error 324 (net::ERR_EMPTY_RESPONSE): The server closed the connection without sending any data.
.
http://pesticider.mediajungle.dk/wp-admin/

As super admin I can't access the site's dashboard either, only the superadmin settings pages.

I also upgraded several themes in our installation yesterday. This particular site uses the theme Blaskan. I thought the theme might be the problem, but other sites running Blaskan seem to be fine (http://embordtennis.mediajungle.dk).

I could try disabling all network plugins, but I am using Pro Sites Plugins to manage plugin availability for local sites (we have aroud 3000 local sites in this installation), so most plugins are already network disabled.

Would you have any suggestions?

Regards
Kristian