Thanks Head_on_a_Stick,
After I created the new user for testing, I still get the same error when trying to access the system settings.
Here is the output of apt-cache policy:
Package files:
100 /var/lib/dpkg/status
release a=now
500
http://security.debian.org/ testing/updates/non-free Translation-en
500
http://security.debian.org/ testing/updates/main Translation-en
500
http://security.debian.org/ testing/updates/contrib Translation-en
500
http://security.debian.org/ testing/updates/non-free amd64 Packages
release o=Debian,a=testing,n=stretch,l=Debian-Security,c=non-free
origin security.debian.org
500
http://security.debian.org/ testing/updates/contrib amd64 Packages
release o=Debian,a=testing,n=stretch,l=Debian-Security,c=contrib
origin security.debian.org
500
http://security.debian.org/ testing/updates/main amd64 Packages
release o=Debian,a=testing,n=stretch,l=Debian-Security,c=main
origin security.debian.org
500
http://ftp.us.debian.org/debian/ testing/non-free Translation-en
500
http://ftp.us.debian.org/debian/ testing/main Translation-en
500
http://ftp.us.debian.org/debian/ testing/contrib Translation-en
500
http://ftp.us.debian.org/debian/ testing/non-free amd64 Packages
release o=Debian,a=testing,n=stretch,l=Debian,c=non-free
origin ftp.us.debian.org
500
http://ftp.us.debian.org/debian/ testing/contrib amd64 Packages
release o=Debian,a=testing,n=stretch,l=Debian,c=contrib
origin ftp.us.debian.org
500
http://ftp.us.debian.org/debian/ testing/main amd64 Packages
release o=Debian,a=testing,n=stretch,l=Debian,c=main
origin ftp.us.debian.org
500
http://ftp.us.debian.org/debian/ jessie-updates/main Translation-en
500
http://ftp.us.debian.org/debian/ jessie-updates/main amd64 Packages
release o=Debian,a=stable-updates,n=jessie-updates,l=Debian,c=main
origin ftp.us.debian.org
500
http://security.debian.org/ jessie/updates/main Translation-en
500
http://security.debian.org/ jessie/updates/main amd64 Packages
release v=8,o=Debian,a=stable,n=jessie,l=Debian-Security,c=main
origin security.debian.org
500
http://ftp.us.debian.org/debian/ jessie/main Translation-en
500
http://ftp.us.debian.org/debian/ jessie/main amd64 Packages
release v=8.1,o=Debian,a=stable,n=jessie,l=Debian,c=main
origin ftp.us.debian.org
500 cdrom://[Debian GNU/Linux 8 _Jessie_ - Official Snapshot amd64 LIVE/INSTALL Binary 20150425-14:07]/ jessie/main amd64 Packages
origin Debian GNU/Linux 8 _Jessie_ - Official Snapshot amd64 LIVE/INSTALL Binary 20150425-14:07
Pinned packages: