It's just like Daniel Barlow wrote in his diary http://ww.telent.net/diary/2003/1/ :
and another useless error message for the collection: Jan 29 15:16:52 eval sshd[878]: PAM rejected by account configuration[9]: Authentication service cannot retrieve authentication info. (In this case, it turns out to mean "when you renamed the user you're trying to ssh in as, you forgot to update the shadow file")
...so I checked /etc/shadow again and realized, that there ought to be 8 (eight) '+' signs behind the colon:
+:::::::: I just had seven and that was causing the error. :-/