VM backup alwayes finished with "the backup failed to back up the requested files(6)" status

Hi all,

I still can't take vm backup for some client inside vshpere 5.5,when job start i recieved the below error's

6/7/2014 5:00:48 PM - Error bpbrm(pid=33007) from client XXXXX: ERR - Error opening the snapshot disks using given transport mode: Status 23
6/7/2014 5:00:49 PM - Critical bpbrm(pid=33007) from client XXXXX: FTL - cleanup() failed, status 6   

6/7/2014 5:02:00 PM - Info bpbkar(pid=0) done. status: 6: the backup failed to back up the requested files

i have a full admin permission in the vcenter,also i upgraded the master server to 7.6.0.2 and  appliance to 2.6.0.2

so please advice.