I can no longer create new sites on our multisite install.

Hi,
I can no longer create new sites on our multisite install.

Whenever I attempt to create a new site, I get the following error
Fatal error: Call to protected method WP_Roles::_init() from context '' in /var/www/webmaster/xxxx/wp-includes/ms-functions.php on line 1382

I have tried both with batch create, and manually and both fail. The weirdest part is that the site looks like it has been created, but when I visit the URL, then an error message shows up warning that the site couldn't be found.

Any ideas what could be the problem?
Thanks