I've seen this error generally when Server Admin is left open for long periods of time on the server itself. It tends to become unstable for no apparent reason. Rarely see it when using the application remotely in my experience. I've also noticed this error when using the server's bonjour name (which I tend to avoid) rather than its fully qualified domain name (which is what I tend to do). Server Admin should never run like a one legged cat and if it does I would double-check DNS first, followed by permissions and/or the OS and/or the system drive's 'health' afterwards. But without being there it would be difficult to diagnose.
Guidelines on how to set-up an OD Master/Replica relationship should be outlined in the Snow Leopard Server Manual:
https://manuals.info.apple.com/MANUALS/1000/MA1180/en_US/OpenDirAdmin_v10.6.pdf
Chapter 4. Failing that I would look at the Open Directory Admin Manual for Tiger Server (10.4) because as I recall the subject is covered in more detail? It's been a while but basically both servers must be exactly the same OS, DNS as it should be, clocks are synced and SSH is enabled. Beyond that root should be used. Should be the same process in Yosemite although I would check with the Advanced Server Manual for 10.10 first:
https://help.apple.com/advancedserveradmin/mac/4.0/#/apdDC3BA60F-246B-43E8-AB24- F6419B32FF6D
It appears to be fairly limited in in the 'how' of OD Master/Replica relationships? Looking at the Server.App for Yosemite I'm thinking it's all done there?
