read only dabase login problem

Hi all,

we have physical standby database. we openned it  read only. 

while users connecting this database  they are getting error below.

ORA-01552: cannot use system rollback segment for non-system tablespace  "AUDIT_TABLESPACE"

SQL> show parameter audit

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
audit_file_dest                      string     .................
audit_sys_operations                 boolean     FALSE
audit_syslog_level                   string
audit_trail                          string      NONE

What is the problem ?  you have any idea about cause of problem ?

Best Regards....