Ubuntu – How to fix a “Unknown configuration key `foreign-architecture’ found in your `dpkg’ configuration files.” error

Always when I install something, I get the following error multiple times:

Unknown configuration key 'foreign-architecture' found in your 'dpkg'
configuration files. This warning will become a hard error at a later
date, so please remove the offending configuration options and replace
them with 'dpkg –add-architecture' invocations at the command line.

When I try dpkg --add-architecture I get:

Unknown configuration key `foreign-architecture' found in your `dpkg'
configuration files.  This warning will become a hard error at a later
date, so please remove the offending configuration options and replace
them with `dpkg --add-architecture' invocations at the command line.

dpkg: error: --add-architecture takes one argument

Type dpkg --help for help about installing and deinstalling packages [*];
Use `dselect' or `aptitude' for user-friendly package management;
Type dpkg -Dhelp for a list of dpkg debug flag values;
Type dpkg --force-help for a list of forcing options;
Type dpkg-deb --help for help about manipulating *.deb files;

Options marked [*] produce a lot of output - pipe it through `less' or `more' !

I've no problems yet, but since it says This warning will become a hard error at a later date I better do something about this. When I search 'foreign-architecture', I find an empty file, containing not a single byte. I somehow can't delete that file.

Please help, it's a kind of creapy…

I use Ubuntu version 12.10

Updated the questions:

cat /etc/dpkg/dpkg.cfg 
no-debsig 
log /var/log/dpkg.log

ls /etc/dpkg/dpkg.cfg.d/
multiarch