Invoking apt-get update led to following error message

#apt-get update
[...]
Fetched 254kB in 7s (35.8kB/s)
Reading package lists... Done
W: There is no public key available for the following key IDs:
9AA38DCD55BE302B
W: GPG error: http://ftp2.de.debian.org unstable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B
W: GPG error: http://ftp.uni-stuttgart.de testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B
W: GPG error: http://ftp.uni-stuttgart.de stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B
W: You may want to run apt-get update to correct these problems

apt-key update did not fix the issue, but manually importing the key worked fine:

:~#apt-key update
gpg: key 2D230C5F: "Debian Archive Automatic Signing Key (2006) <ftpmaster@debian.org>" not changed
gpg: key 6070D3A1: "Debian Archive Automatic Signing Key (4.0/etch) <ftpmaster@debian.org>" not changed
gpg: key ADB11277: "Etch Stable Release Key <debian-release@lists.debian.org>" not changed
gpg: Total number processed: 3
gpg:              unchanged: 3

:~# gpg --keyserver wwwkeys.eu.pgp.net --recv-keys 9AA38DCD55BE302B
gpg: key 55BE302B: public key "Debian Archive Automatic Signing Key (5.0/lenny) <ftpmaster@debian.org>" imported
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)
:~# apt-key add ~/.gnupg/pubring.gpg
OK
:~# apt-get update
[...]
Fetched 2702B in 2s (991B/s)
Reading package lists... Done

Alternatively do this

:~# gpg --keyserver wwwkeys.eu.pgp.net --recv-keys 9AA38DCD55BE302B
gpg: key 55BE302B: public key "Debian Archive Automatic Signing Key (5.0/lenny) <ftpmaster@debian.org>" imported
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)
:~# gpg –export –armor 9AA38DCD55BE302B | apt-key add -
OK