Server.app in Lion is a pretty good app for most tasks. But I find myself frequently doing things that I don’t think developers intended me to do. One such item is setting up and tearing down Open Directory to test various iterations of enabling a master. I frequently use slapconfig to destroyldapserver:
Doing so almost immediately allows me to demote an Open Directory master to a stand-alone server and then repromote the server to a master or replica for testing purposes. If you do this, then Open Directory cannot be set back up using Server.app. The fix is to use Server Admin to repromote your server back to an Open Directory master and then use Server Admin to more graciously demote the server back to stand-alone. Until you do this, the Server.app will error out on Open Directory promotions that the server is already an Open Directory master.
A change I’ve made to my workflow when nukin’ and pavin’ OD is to just use Server Admin for the paving part. If you demote with Server Admin you won’t have these issues. Hope this helps someone who finds similar wonkiness.