ORA-01552

ORA-01552:cannot use system rollback segment for non-system tablespace 'USERS' Oracle.
I was getting this error in our oracle release 10.2.0.0. Main cause I found was that somehow I have changed parameter undo_management = manual that by default was auto.
I just changed undo_management = auto and than all problems with rollback segments disappeared.
sql> alter system set undo_management = auto scope = spfile;
sql>shut immediate
sql>startup