"could not move to library" in lw 4.2.6 in os x.

i've been using that version for some time and never seen "could not move to library" ever before. now within the space of about three days i see it twice.

after it has occured downloads.dat a downloads.bak are gone, rendering all my partial downloads useless: hours of dowloading time wasted -- something lw's good at doing. does anyone know what's going on with that?

what on earth would make it suddenly start doing that? the first time it happened i deleted all lw pref files.