ora 27206 during rman backup

I am using RMAN with Legato Network admin for backups to tapes.
Rman backup for last night terminated with the foll error messages, it has been running fine before:

RMAN-08013: channel t1: piece 1 created
on 3.0.0.0: piece handle=df_t453593752_s265_p1 comment=API Version 2.0,MMS Versi
RMAN-08525: backup set complete, elapsed time: 00:03:35
RMAN-08008: channel t1: starting incremental level 2 datafile backupset
RMAN-08502: set_count=266 set_stamp=453593967 creation_time=11-FEB-02
RMAN-08010: channel t1: specifying datafile(s) in backupset
RMAN-08522: input datafile fno=00017 name=/ora_lob/tcmslob/lob3.dbf
/rbs03.dbf: input datafile fno=00015 name=/ora_rbs/orainventory/oradata/babprod1
RMAN-08013: channel t1: piece 1 created
on 3.0.0.0: piece handle=df_t453593967_s266_p1 comment=API Version 2.0,MMS Versi
RMAN-08525: backup set complete, elapsed time: 00:01:55
RMAN-08008: channel t1: starting incremental level 2 datafile backupset
RMAN-08502: set_count=267 set_stamp=453594082 creation_time=11-FEB-02
RMAN-08010: channel t1: specifying datafile(s) in backupset
RMAN-08522: input datafile fno=00019 name=/ora_lob/tcmslob/lob4.dbf
RMAN-03026: error recovery releasing channel resources
RMAN-08031: released channel: t1
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03006: non-retryable error occurred during execution of command: backup
RMAN-07004: unhandled exception during command execution on channel t1
a management catalogn raised in RPC: ORA-27206: requested file not found in medi
ATEN-10031: ORA-19583 occurred during call to DBMS_BACKUP_RESTORE.BACKUPPIECECRE

What is the cause for this error message, didn't find much documents and all are related to using VERITAS net backup.
Any clues, please help..

THanx a lot