Thanks for your suggestions, Carl and Clive, everything seems to be fine with the keyboard mapping. I tried to log in at a tty (in text mode). After entering the username (wahler), the message "Login incorrect." appeared. No password was being asked for, and the login prompt reappeared again. /var/log/auth.log witnessed the whole thing with login[25098]: FAILED LOGIN (1) on `tty2' FOR `wahler', Authentication failure