When you validate a source
Please open /etc/apt/sources.list by text editor in root authority.
sudo leafpad /etc/apt/sources.list
When you use MATE, please change "leafpad" to "pluma".
Please find the line:
deb-src http://ftp.hr.debian.org/debian/ wheezy/updates ...
Please change this as follows:
deb-src http://ftp.hr.debian.org/debian/ wheezy-updates ...
The line of security.debian.org, please be careful not to change it.
After having performed this, please confirm that you work definitely.
When you invalidate a source
Please run Synaptic, and choose Settings - Repositories.
Please exclude a check of Source code.
After having performed this, please confirm that you work definitely.