Pingback not working

I recently upgraded to 3.8.1 (multisite), and since then, my users have not been able to successfully use the pingback feature. I have the option selected on all sites, but when someone publishes a post that has a link to another post in another site on the same WordPress MS installation, the pingback never shows up. I’ve checked the PHP error logs, and the Apache logs.

I have disabled all plugins and reset the theme to Twenty Ten.

Any help would be greatly appreciated ??