Debian Configuration :: Upgraded 7.8 Aetheros AR9565 WiFi Card And No RDP

Because of workers here in my company I need a very secure operating system for laptops so I have chosen Debian 7.8. The laptop is Acer Aspire E11 (E3-112-C4NE). This laptop has a built-in wifi card Aetheros AR9565 :

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lspci -nn
02:00.0 Network controller [0280]: Atheros Communications Inc. AR9565 Wireless Network Adapter [168c:0036] (rev 01)
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Basically the stable 7.8 distribution cannot handle this wifi card, so I read on forums that I should upgrade my operating system. So I did as I read on the forum:

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nano /etc/apt/sources.list
deb http://ftp.debian.org/debian wheezy-backports main contrib non-free
apt-get update
apt-get -t wheezy-backports install firmware-atheros
apt-get -t wheezy-backports install linux-image-amd64
reboot
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After this the oprating system recognized the wifi card and loaded the "ath9k" driver. In the Grub the default operating system became :

Linux 3.16.0-0.bpo.4-686-pae

Because I want to restric the workers to access anything else except the designed inner network, I have uninstalled the Network Manager and set the network interfaces in the "/etc/network/interfaces" file as shown below:

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# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface

auto lo
iface lo inet loopback

[Code] ....

This works, when the operating system boots, it automatically connects to my hidden SSID wiofi network. I can ping other computers on the same IP subnet (10.139.252.xxx).

This computer should connect to an XP running under virtualbox, having the IP address of 10.40.138.153. But it does not. On another laptop, running Ubuntu 14.05, having the same network configuration, (10.139.252.102) connects to the same XP under virtualbox with no problem....

For Ubuntu nothing is required to be installed on virtualbox, no extension pack, no guest addition, nothing. When I want to connect to XP via rdp, it asks for username and password and after that only a blue or black blank screen and nothing else... Even on the XP I see no sign of any activity that somebody would like to connect.

Checking the network traffic (tcpdump, nast) I see that the two computers are communicating but after a while the communication stops. The rdp XP its in another IP subnet, but Ubuntu can connect without any problem, Debian not...