The repository-key expiration date has been extended recently. The expiration date of the renewed key has been set to be October 3rd, 2013. You will have to update the key in your apt keyring by either using (e.g.) the pgp.mit.edu keyserver:
apt-key adv --keyserver subkeys.pgp.net --refresh-keys E394D996
or a local keyfile in ASCII format at wgdd_archive_key.asc
wget -O - http://debian.wgdd.de/stuff/wgdd_archive_key.asc | apt-key add -
or by updating the wgdd-archive-keyring Debian package. Detailed information can also be found here.
