I created a test site and couldn't replicate the problem.
I upgraded the production site to 4.2.4 and the menus display fine, but I can't save changes to the navmenu without getting an immediate server 500 error.
The only changes I've made recently to the site are upgrading to Wordpress 3.4.1 and Suffusion 4.2.4.
There is nothing in my PHP error logs, nor Apache error logs. The Apache access logs show the 500 error, but no root cause. It doesn't appear PHP is running out of memory, or that there is a timeout. I'm not sure if this is a possible Wordpress or Suffusion issue.