Site gone after WordPress 3.4 update

After I updated my multisite installation – http://www.naergilien.info and http://blog.naergilien.info – my site is practically gone, meaning, when I try to access either of the sites or the admin panel, all I get is a blank page.

I’ve already tried the manual update; the result is the same.
Since I can’t access the admin panel, I also tried setting the theme (I used suffusion) to twentyeleven using the options table in the database. Result: same; still blank pages.

The error.log shows this (paths shortened for security reasons; so those aren’t the ‘real’ paths at the beginning, but you get the point):

[14-Jun-2012 08:30:56 UTC] PHP Fatal error:  Cannot redeclare wp_is_mobile() (previously declared in /naergilien/wp-includes/vars.php:107) in /naergilien/wp-includes/vars.php on line 126

Uhmmm… *help* please?