Backup fails when pre/post commands are set

I'm using on a desktop ATIH 2014 Build 6673 under Windows 8.1 Update 1 to make a backup from a folder and its sub folders to a share on a NAS each day at 22:00 by waking up the hibernated desktop. - It does what it should, it wakes up the hibernated desktop and makes the backup.

However I have a little problem with this solution, the desktop will be don't hibernated after the backup is done. To solve this problem I have written a batch to hibernate the desktop which I wanted to run as post command from the backup task. When I will configure pre/post commands I have also to define a Windows account (I'm using an account which have administrator rights on the desktop), but in this case the backup fails with following error:

Event details
Access to the specified file is denied: \\?\UNC\192.168.0.252\Backup\VEISZ-PC\FileBackup_Data\Vallalkozas\Vallalkozas_inc_b3_s6_v1.tib.

Error Code
More information about this error and solutions may be available online in the Acronis Knowledge Base.
To access the online resource manually, enter the event code at: http://kb.acronis.com/errorcode/
Event code: 0x00040014+0x00040014+0x0000FFF0+0x80070005

What I'm doing wrong? The file which will be accessed on the NAS (192.168.0.252) does exists. I have modified the same backup task which runs successfully when I don't define pre/post commands and the Windows account.