I can't explain how the days to keep incomplete setting change to zero, but you may be able to resume that incomplete file since you saved the folder. Try using 21 before you quit LW.

If the saved incomplete folder contains a "downloads.dat" file, just switch the saved and the current incomplete folder locations when LW is quit.

The next time Limewire starts, it will read the downloads.dat file which contains the index of all the incomplete pieces, and you should be able to resume the incomplete from the Library.

If the downloads.dat file that goes with the incomplete file is gone though,then the old incomplete cannot be resumed.

best wishes