I had a bug which presented itself in the same way. I looked at
sources.list and found that the lines
deb http://ftp.gb.debian.org/debian/ squeeze main
deb-src http://ftb.gb.debian.org/debian/ squeeze main
were present. I checked the mirrors listed on the Debian website
http://www.debian.org/mirror/list
and found that the UK mirror was listed as
http://ftp.*uk*.debian.org/debian
under "Great Britain". I changed the addresses in my sources.list and
software-properties-gtk loaded fine from the command line and the
administration gui menu. I had not set these values so I can only assume
that Debian Squeeze installs the wrong UK mirrors currently.
I hope this helps,
James
