I got the following error message when trying to connect to rman:

/u01/oradba/rman/config > rman

Recovery Manager: Release 10.2.0.3.0 - Production on Thu May 21 14:14:01 2009

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

RMAN> connect target /

connected to target database: PROD (DBID=1824257732)
RMAN-06900: WARNING: unable to generate V$RMAN_STATUS or V$RMAN_OUTPUT row
RMAN-06901: WARNING: disabling update of the V$RMAN_STATUS and V$RMAN_OUTPUT rows
ORACLE error from target database:
ORA-19921: maximum number of 64 rows exceeded

I went online and every where searching for it but couldn't find a solution and then i immediately remembered that i had my laptop connect at home with another session that was already connected to rman.  So i checked out ps command to see any other rman command connected and yup i was right.  I killed it:

/u01/oradba/rman/config > ps -aef|grep rman
  oracle   165 22875   0 21:45:59 pts/8       4:41 rman
  oracle 19379  6839   0 14:14:54 pts/13      0:00 grep rman

prodnj54db1*PROD-(oracle)
/u01/oradba/rman/config > kill -9 165

prodnj54db1*PROD-(oracle)
/u01/oradba/rman/config > rman

Recovery Manager: Release 10.2.0.3.0 - Production on Thu May 21 14:15:04 2009

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

RMAN> connect target /

connected to target database: PROD (DBID=1824257732)

RMAN> connect rcvcat rman/rmanpasswd@rmancat

connected to recovery catalog database

RMAN>

The kill command did the trick.

Hope this helps.  Do let me know if this helped you out.

===========================================================

Oracle Error :: RMAN-06901

WARNING: disabling update of the V$RMAN_STATUS and V$RMAN_OUTPUT rows

Cause

Informational message only.

Action

No action required.

RMAN-06900:   WARNING: unable to generate V$RMAN_STATUS or V$RMAN_OUTPUT row

RMAN-06901: WARNING: disabling update of the V$RMAN_STATUS and V$RMAN_OUTPUT rows
ORACLE error from target database:
ORA-19921: maximum number of 64 rows exceeded

The main thing about this error is that it's relate to how many concurrent sessions you have.  I was already logged in manually to run a backup of archive logs manually and we had another script that run the backup of archive logs via cron.  So when the script in cron kicked off it received this message .   This is how i found out that when you have more then 1 connection this is why you get the message.

# oerr rman 6900
6900, 3, "WARNING: unable to generate V$RMAN_STATUS or V$RMAN_OUTPUT row"
// *Cause: The routine createRmanStatusRow() or createRmanOutputRow()
// could add new row into V$RMAN_STATUS or V$RMAN_OUTPUT.
// *Action: Check the associated error messages. If the associated error
// message indicates a condition that can be corrected, do so,
// otherwise contact Oracle.

# oerr ora 19921
19921, 00000, "maximum number of %s rows exceeded"
// *Cause: The maximum number of rows in the V$RMAN_STATUS or V$RMAN_OUTPUT
// table has been exceeded.
// *Action: Close some of existing and unused RMAN connections and sessions.

The specific Bug is 4659734.   I believe this has been fixed in 11g.

Hope this helps.