Thanks to all just for reading up to this point. :) I hope I'll be able to thank you once more for any ideas, pointers...
(Well am I so bad at Googling or what? I always start to find reasonable sources *right after* I ask.) There's a debate on this error, though the bug is originally about this message causing login failures: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=578937 Questions remain yet unanswered, though...