I have found a solution. The file /var/lib/debtags/vocabulary is missing. "sudo debtags update" solves the problem.
This command is commented out in the debtags.postinst-file. -- [Dapper] Adept can't open APT database https://launchpad.net/bugs/40410 -- kubuntu-bugs mailing list kubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/kubuntu-bugs
