Problem when upgrading automatically

Hi,
when trying to run the automatic upgrade process from 3.1.2 to 3.1.3, there is a new page “Update WordPress” with the message “Downloading update from http://wordpress.org/wordpress-3.1.3.zip…” and it seems to “freeze” immediately, nothing happens at all. That’s the first time, all other upgrades worked fine.
Any hint?
BR