Bad signature for updated packages

[Jochen Antesberger]

I just did my ususual aptitude update session to install security 
updates. Turns out that ntpdate updates fine, but the other three 
candidates, gdm, devicekit-disks and seahorse-plugins cause a warning 
that these packages are not trusted.

Is there a problem on the Ubuntu servers (using the german mirror) or do 
I have a corrupted file transfer there?

How am I going to fix this one, please?

Jochen