Thank you KawateTadako! And: After downgrading to 2.2.7 it indexed in few minutes all what it has tried to index for hours (if not days) and it is syncing as a charm.... (OSX El Capitan 10.11.3 here)
Can anyone point to a safe link to download 2.2.7? BitTorrent Sync official site has AFAIK only 2.3.3 and - frankly - should remove it soon or at least leave older versions available. I thought this problem was due to recent upgrade to El Capitan but I see is a bug everywhere.