This happens only sometimes!
1. Copy an Object DN
2. Past it to the parent
3. Rename it
4. While saving something goes wrong with connection for example / force a tree refresh / newly created item is NOT visible in the tree.
5. Repeat step 1 - 3
6. Error: entry already exists but the item is definitely not a member of the tree.
I did not found any way to fix this. Even after removing the cache. No way.