It seems difficult to believe that it is as you suspect, that something that a client did has locked you out of control of your own server.
Check permissions! Break out the Terminal and attempt to discover where the critical configuration files are for Open Directory, and make sure your permissions (
ls -l ) haven't been "bonked" <-- (sorry for the inscrutable admin rhetioric ;-)
Also, if you find the config file(s) in question, you will be able to examine and edit manually (
sudo cat ____ and
sudo vi ____). If these files are in an xml binary form, they can be converted with
plutil -convert xml1 config.plist and converted back with
pluitil -convert binary1 config.plist -- as always, copy the orginals somewhere else, back them up, and work with copies to experiment, so you can always restore them necessary.
also you can attempt to use the
serveradmin (<---that's hotlinked) command line tool to attempt what the GUI is preventing you from doing. here is another good reasource, as well as here
It is difficult to diagnose/troubleshoot this without having access to the machine, so I hope I give you ideas to try that may lead to new paths to a solution. Again, let me know in comments if this answer is not helpful, and I will again delete to prevent noise. Good luck!