This error is seen the second time a local database repair in DSREPAIR is run.
If you check with DSBROWSE, the Tree Root object has a class of "Invalid ID". If you look at the attributes of the Tree Root object, you would also notice that the T attribute (naming attribute for the Tree Root class) is missing.
If you run DSREPAIR again, and perform a local database repair before the backlink process has been executed, the error message will appear notifying you that the ClassID field will be cleared.
The reason for this extra step in DSREPAIR is to check if the information stored in the external reference entry is correct. Part of what needs to be corrected is that the naming attribute (the T attribute) needs to be added. It is possible that this attribute will not be added, thus if you were later to run DSREPAIR twice in a row you'd see this message again. This is a cosmetic issue and should not affect the correct working of your server.
If you wanted anyway to add it, the easiest way is to add a replica of the Root partition to the server and then take it away. You should then see the T attribute in the external reference object.
Formerly known as TID# 10097632
Formerly known as TID# NOVL102061