Hi,
I faced the same problem. Even I removed and recreated the schema instance I got errors in sde_setup.log file as shown below:
[Mon Oct  7 08:16:12 2013] Error creating  GDB_Tables_Last_Modified table... 
[Mon Oct  7 08:16:12 2013] ERROR Creating Geodatabase tables
, Error = -51
,EXT_Error = -942
,EXT_ERROR1 = Error executing stored procedure sde.instances_util.check_instance_table_conflicts
,EXT_ERROR2 = ORA-00942: table or view does not exist
After deleting the orphan record from SDE.INSTANCES related to deleted instance, the problem was solved.
Thanks a lot!
Sincerely.