I created a file with the contents mentioned in comment #23, but this hasn't solved it. Instead I get the following error message now when I install or remove any package:

Entfernen von libavahi-ui-gtk3-0:amd64 ...
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.

Entfernen von libgvc5 ...
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.

Entfernen von libgraph4 ...
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.

Entfernen von libcdt4 ...
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.

Entfernen von libchamplain-gtk-0.12-0:amd64 ...
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.

Entfernen von libchamplain-0.12-0:amd64 ...
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.

Entfernen von libstreamanalyzer0 ...
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.

Entfernen von libsoprano4 ...
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.

Entfernen von libclucene0ldbl ...
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.

Entfernen von libdlrestrictions1 ...
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.