Every 7 days linux cannot mount shared folder

[nanotek]

I have a TS-431X nas and a server mounting centos 6.
All is updated daily (if there are update) .
Server can reach every time the nas (ping -2 is executed every 10 minutes and logged).
On the nas the compatiblity is set to SMB = 2,1.
I have the problem from about two months.

Every 7 days the server is unable to mount the shared folder and give the error:

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Error while mounting 192.168.1.180:Backup : mount error(5): Input/output error Refer to the mount.cifs(8) manual page (e.g. man mount.cifs) 

the mount is done by executing this command:

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mount -t cifs -o username=user,password=pass //192.168.1.180/Backup /mnt/backup

Using this command manually, also give the same error.

I've noticed that, subsequently to the error, if i mount the shared folder on a windows pc (mount is executed without problem) surprisingly the server is now able to mount the sherd folder !

There is any authentication period to set on QTS firmware ?
Anyone have had this problem ? (The problem shows up if the shared folder is used only by linux systems for more than 7 days)