Database Software

Hi, all,
I have a strange problem. Right now I have 2 node RAC primary on Windows and another 2 node RAC standby (physical standby) also on Windows 2003 (R2). I did a few times to switch the standby from redo apply to read-only without any problems. Today, i tried the switch and it kept giving me the error as follows:

SQL> alter database recover managed standby database disconnect;

Database altered.

SQL> alter database recover managed standby database cancel;
alter database recover managed standby database cancel
*
ERROR at line 1:
ORA-16136: Managed Standby Recovery not active

I also check the data guard broker and archive logs and everything looks fine. All the redo logs are applied to standby. At one time, the statement of alter database recover managed standby database cancel works, but the next alter database open statement crashed the instance.

Any suggestions?