kmail does not find backend for listing encryption-keys

Hello! I just noticed that my KDE 3.5.3-kmail does not offer encrypting emails anymore, which did work before. I had a look at the settings for the user and found that the assigned key from 3.5.1 was gone, so I tried to add it again. I am then told that kmail could not find any backends for listing the encryption keys. Since I did not delete any packages, but just updated, I wonder what caused this error. What packages could be missing in order to cause this error? Sven