You guys have likely already figured this one out by now, but--- if you haven't....
Double check your server settings. I just ran into this same problem, and after revalidating files, wiping, and reloading, I discovered it was simply a mistake on my entry in the Blood Moon Horde Zombies speed setting.
In your Serverconfig.xml file (or whoever owns the server and has access to said XML) somewhere within that will be:
Zombie Settings
<property name="ZombieBMMove" value="x"
You gatta change that value to a number that your Server recognizes--- otherwise, they will just stand there as they have no speed input (or in this case) IndexOutOfRange.
Change this to 0-4 (0 for walk, 1 for jog, 3 for sprint, 4 for nightmare) and your Blood Moon Zombies should be back to their regular bloodthirsty and ravenous selves.
Hope it helps.