Sometimes it looks like your ldap db is corrupted.
In that case you can try to re-index your ladp
1. Stop slapd with
sudo launchctl unload /System/Library/LaunchDaemons/org.openldap.slapd.xml
2. Wait a minute to give slapd time to stop. You can see it in the OD status pane of Server Admin
3. Re-index your ladp db with
sudo slapindex -v -d 1
(I like the debug output just to see what it is doing.)
4. Start slapd back up with
sudo launchctl load /System/Library/LaunchDaemons/org.openldap.slapd.xml
See if that helps.