The name you're trying to use might be a banned name. They're constantly updating the list of banned names so it's possible for someone to already a name on one server, but is now banned from creation on another server (and those people would likely have a problem if they ever moved server and/or renamed and tried to rename back).

Unfortunately they don't publish the banned names list, but if you're trying to make a name of something used elsewhere, that might be why.