This happened to me, but may not be your case...
Mgrating to 5.2 the I just copied the 99user.ldif file over and I ended up with two 'schemacsn' attributes (or something like that) at the very end of the file. The attribute itself is single valued to begin with, and also, I believe it keeps track of the changes, therefore the csn assigned when I tried to add anew attribute did not match. Bottom line:
1. Stop your directory server.
2. edit the 99user.ldif file and delete the schemacsn attribute (or attributes) --they may be at the end of the file.
3. Restart the directory.
4. Add attributes to your heart content...