Can't add new blog on multisite after update to WP 3.1

Since I updated to 3.1 my multisite is giving me a funny (well, not really) answer when I try to install another subdomain blog.

After the usual procedure there is a white screen that asks me “Do you really want to do this?” As the only choice for an answer it gives me a link labelled “Try again”. That link gets me back to the network admin dashboard but the new blog doesn’t appear.

It says this in my .htaccess

# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule . index.php [L]
# END WordPress

and this in wp-config

define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', true );
$base = '/';
define( 'DOMAIN_CURRENT_SITE', 'www.smmp.de' );
define( 'PATH_CURRENT_SITE', '/' );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );
define( 'SUNRISE', 'on' );

Deactivating all networked plugins didn’t help.

Any ideas why I can’t add a new blog?