thanks for your fast answer - I did a downgrade of maildrop, and it fixed my problem!
For the history:
from the aptitude logs: courier-maildrop 0.60.0-2 -> 0.63.0-2 - so i removed the unstable sources and reinstalled courier-maildrop with that version.
After i entered "postfix flush" I even got all my emails
You made my day!