versuch mal
apt-get -f update
Hit http://security.debian.org squeeze/updates Release.gpg
Ign http://security.debian.org/ squeeze/updates/contrib Translation-en
Ign http://security.debian.org/ squeeze/updates/contrib Translation-en_US
Ign http://security.debian.org/ squeeze/updates/main Translation-en
Ign http://security.debian.org/ squeeze/updates/main Translation-en_US
Ign http://security.debian.org/ squeeze/updates/non-free Translation-en
Ign http://security.debian.org/ squeeze/updates/non-free Translation-en_US
Hit http://security.debian.org squeeze/updates Release
Hit http://ftp2.de.debian.org squeeze Release.gpg
Ign http://ftp2.de.debian.org/debian/ squeeze/contrib Translation-en
Ign http://ftp2.de.debian.org/debian/ squeeze/contrib Translation-en_US
Ign http://ftp2.de.debian.org/debian/ squeeze/main Translation-en
Ign http://ftp2.de.debian.org/debian/ squeeze/main Translation-en_US
Ign http://ftp2.de.debian.org/debian/ squeeze/non-free Translation-en
Ign http://ftp2.de.debian.org/debian/ squeeze/non-free Translation-en_US
Get:1 http://ftp2.de.debian.org squeeze-updates Release.gpg [836 B]
Ign http://ftp2.de.debian.org/debian/ squeeze-updates/contrib Translation-en
Ign http://ftp2.de.debian.org/debian/ squeeze-updates/contrib Translation-en_US
Hit http://packages.dotdeb.org squeeze-php54 Release.gpg
Ign http://packages.dotdeb.org/ squeeze-php54/all Translation-en
Ign http://packages.dotdeb.org/ squeeze-php54/all Translation-en_US
Ign http://ftp2.de.debian.org/debian/ squeeze-updates/main Translation-en
Ign http://ftp2.de.debian.org/debian/ squeeze-updates/main Translation-en_US
Ign http://ftp2.de.debian.org/debian/ squeeze-updates/non-free Translation-en
Ign http://ftp2.de.debian.org/debian/ squeeze-updates/non-free Translation-en_US
Hit http://ftp2.de.debian.org squeeze Release
Hit http://security.debian.org squeeze/updates/main Sources
Get:2 http://ftp2.de.debian.org squeeze-updates Release [113 kB]
Hit http://security.debian.org squeeze/updates/contrib Sources
Hit http://security.debian.org squeeze/updates/non-free Sources
Hit http://security.debian.org squeeze/updates/main amd64 Packages
Hit http://security.debian.org squeeze/updates/contrib amd64 Packages
Hit http://security.debian.org squeeze/updates/non-free amd64 Packages
Hit http://packages.dotdeb.org squeeze-php54 Release
Err http://ftp2.de.debian.org squeeze-updates Release
Hit http://ftp2.de.debian.org squeeze/main Sources
Hit http://ftp2.de.debian.org squeeze/non-free Sources
Hit http://ftp2.de.debian.org squeeze/contrib Sources
Hit http://ftp2.de.debian.org squeeze/main amd64 Packages
Hit http://ftp2.de.debian.org squeeze/non-free amd64 Packages
Hit http://ftp2.de.debian.org squeeze/contrib amd64 Packages
Hit http://packages.dotdeb.org squeeze-php54/all Sources
Hit http://packages.dotdeb.org squeeze-php54/all amd64 Packages
Fetched 837 B in 0s (8,989 B/s)
Reading package lists... Done
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://ftp2.de.debian.org squeeze-updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553
W: Failed to fetch http://ftp2.de.debian.org/debi…s/squeeze-updates/Release
W: Some index files failed to download, they have been ignored, or old ones used instead.