RMAN datafile not found in recovery catalog RMAN 20201

Hi ALL;
I have set up rman for backups. Been working fine. Then a few days ago it fails with this error below. I think that the file_id, or file# (10000) refers to a datafile that is out of sync with the recovery catalog. But the problem is I don't have any datafile with that file_id or file#. Infact I don't have up to 500 datafiles let alone 10000. So a resync catalog doesn't help. I even have a resync cron job in place long before this. I had added some datafiles but none of them has a file_id of 10000. The question is Where did Rman get that file_id (10000) from? Or does it really refer to a file_id at all?

RMAN-06004: ORACLE error from recovery catalog database: RMAN-20201: datafile not found in the recovery catalog
RMAN-06010: error while looking up datafile: 10000

Please help
Thanks
Tony