You must use the reset database command in RMAN to create a new database incarnation record in the recovery catalog.

% rman connect target user/pwd catalog user/pwd
RMAN> reset database;

Immediately back up the database. This will give the database a new incarnation.

Explanation:
If you issue the ALTER DATABASE OPEN RESETLOGS but do not reset the database, then RMAN will not access the recovery catalog because it cannot distinguish between a RESETLOGS command and an accidental restore of an old control file. By resetting the database, you inform RMAN that the database has been opened with the RESETLOGS option