Hi,
Please explicitly resync the recovery catalog through the command
RMAN>resync catalog;
If after explicitly syncronising the database the same error occurs while taking the backup see that the recovery catalog is created in user RMAN or other user and not in SYS and SYSTEM.
In case of any help please be free to ask me at [email protected]

Regards,
ROhit Nirkhe,Oracle DBA,OCP 8i
[email protected]