Hi,
Try with the official Twenty Twelve Theme. If all is ok, the problem is with your Theme.
I now tryed with Twenty Twelve and with my custom Theme, all is ok.
Maybe the wp-content directory of WordPress is not writable. WordPress can not create uploads directory.
This is an error in plugin. The WordPress version 3.5 generate this error Warning: Missing argument 2 for wpdb::prepare(), called in… The second argument must be the value for % in prepared statement.
On your server the php mod_rewrite function enabled?
Is the .htaccess file in wordpress root directory created(or writable)?
