well, i fear i found the reason... <a href="http://savannah.gnu.org/bugs/?group_id=1409" target="_blank">http://savannah.gnu.org/bugs/?group_id=1409</a> -- look at bug #419 and #538...
i deleted the entire mldonkey-distrib directory (well, not the incoming directory, of course!) and re-extracted the binaries again. lost my started downloads, but it works again.