When you download the torrent file from the site, AFAIK it generally helps keep track of the download history of the file. That was why I suggested opening that original torrent file. If that doesn't work then yes, try downgrading back to LW 4.
LW 4 will probably not be as good with torrents, however if you follow the instructions carefully in that link your downloads should all restart. Only issue with downgrading to LW 4 however is although files will restart, they will show zero 5. I am not sure about torrents however. After downgrading, open the original torrent file with LW (not the fresh one you downloaded from the site, trash the newer one.)