Hope this isn't too late, or that at least it is useful for others.
I tried http://support.apple.com/kb/ht1194 several times to no avail. I was simply unable to authenticate with the diradmin account.
An Apple support person recommended:
- Archive the OD from the Server.app → OD panel → action menu → Archive Open Directory Master…
- Select the server in the list and click on the minus button Once the OD has been destroyed, You will see the OD is now Off.
- Create the OD by switching back to ON and create the OD again with the same information as before (including the DirAdmin)
- Create a BackupDirAdmin account with WGM to have a backdoor, just in
- In terminal restore the archive you saved in step 1 using
sudo slapconfig -restoredb < archive-path >
- Retest the authentication after the directory has been restored.
Try as I might I couldn't get this to work, and then I noticed that in Mountain Lion server the slapconfig command had lost the -merge option so restoring the database was dumping both the new DirAdmin and backdoor account....
Bizarrely, after a lot of digging around I saw that mkpassdb has a -setadmin flag, so I tried to use this to elevate another slotId to admin rights. This worked, and then I was able to reset the password for the DirAdmin account.