I am using 10.1.0.3.0 and facing the following issue:

RMAN> backup format '\\10.91.0.49\testdb_backup\backup\data\%U.bak' datafile 1;

Starting backup at 24-MAR-11
allocated channel: ORA_DISK_1
channel ORA_DISK_1: sid=1077 devtype=DISK
channel ORA_DISK_1: starting full datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
input datafile fno=00001 name=E:\IDRIVE\DMSW\SYS1DMSW.ORA
channel ORA_DISK_1: starting piece 1 at 24-MAR-11
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03009: failure of backup command on ORA_DISK_1 channel at 03/24/2011 21:06:39
ORA-19504: failed to create file "\\10.91.0.49\TESTDB_BACKUP\BACKUP\DATA\0VM826IU_1_1.BAK"
ORA-27040: file create error, unable to create file
OSD-04002: unable to open file
O/S-Error: (OS 5) Access is denied.

I have used the network paths before too but no on 10.1.0.3.0, I used them on 10.2.0.x.

The path on which the file cannot be created exists and I am not able to create test files on it. Checked on web but could not find anything relevant to my case. Any suggestions?