As it is so easy to remove and reinstall, why not remove clamtk and see what happens?
Meanwhile I did that but soon discovered a much bigger problem. Directly after deinstallation it did not change anything, and I detected, that it also affects windows-exe files, which should be started by WINE.
I than just installed the latest update using mit-update (it was something concerning hal) and rebooted, to see whether that helps.
After rebooting I was able to log into my account but right after logging in the boot process stopped with nothing more than showing me the mouse pointer. By using a predefined keyboard shortcut I'm able to start the system monitor, but nothing more.
Is there a way to get back to normal? I don't even know, what was causing this accident.
Should I open a new post, since this seems to be not related to the original question?