It happens to me if I "overload" traffic. If I open few tabs with various sites in browser (after download started) or run some online game or torrent, in 80% cases nopy download fails. No such problems with other sites (wokrupload, zippy, mega, etc - can open whatever I want, if it's too much than download just lasts a little longer, but no fails), so I use nopy only as last resort and once download started, don't open anything else.