qBittorrent official forums

Hello.
After update (from 4.1.0 to 4.1.5) I lost my torrents (was ~400). Now I have ~130.
qBittorrent\BT_backup folder contain ~700 .torrent files and ~400 .fastresume files (many of them have 0Kb size).
How I can restore my downloads?

I remembered.
Sometimes I made a backup of 'BT_Backup' folder.
Now I recovered the files (.torrent and .fastrestore). It seems all right.
But how to avoid this in the future?