ORA-1545 during DROP ROLLBACK SEGMENT <rbsname>

ORA-1545 may be signalled during a "DROP ROLLBACK SEGMENT" if the
rollback segment is currently online.
eg: SVRMGR> drop rollback segment rprbs;
drop rollback segment rprbs
*
ORA-01545: rollback segment '' specified not available
Actions:
1. Check the status of the rollback segment in DBA_ROLLBACK_SEGS.
eg: SELECT segment_id, status FROM dba_rollback_segs
WHERE segment_name = '';
Note the SEGMENT_ID and STATUS for the steps below.
2. If the STATUS is ONLINE then:
ALTER ROLLBACK SEGMENT  OFFLINE;
DROP ROLLBACK SEGMENT ;
If this still reports ORA-1545 continue with step 3 below.
3. ORA-1545 may also be signalled if there is a problem reading
the rollback segment header information or if the rollback segment
contains active or in-doubt transactions. Note the SEGMENT_ID and
SEGMENT_NAME and go to "Recovering Rollback Segments"