Unable to remove a user in Oracle 11g
The ORA-01403 seems to indicate that you have a System Event Trigger for DROP USER.

You'll need to hunt down the appropriate trigger.

SELECT *
from dba_triggers
where base_object_type like 'DATABASE%'
and triggering_event like 'DROP%'
and status='ENABLED'
;


If that doesn't work, then I recommend that you open a ticket with Oracle Support.