i've found out kind of a way of dealing with this, in case anyone else suffers from this problem. it doesn't tackle the already unresumeable files, but it is a way of avoiding, or at least getter round the problem:

keep manually duplicating the "downloads.dat" file in your downloads folder in the finder at useful moments (like after something completes or after you add a file to the dl list) and if and when the problem strikes, delete the now corrupt downloads.dat file and rename your last duplicated back to "downloads.dat" and the download list, so therfore the resumability of your partially downloaded files, is rescued.

still can't resume previous partial downloads that happend before i started doing the above though