I have looked into ItemRack.lua and pinpointed that ItemRack.OnEvent() does not receive the "PLAYER_LOGIN" event if PitBull is active. After disabling PitBull everything works fine again. If I manually trigger the "PLAYER_LOGIN" event by "/script ItemRack.OnEvent("PLAYER_LOGIN")" Itemrack works fine even with PitBull loaded. I have absolutly no idea why ItemRack.OnEvent() doesn't get the "PLAYER_LOGIN" event at loading time if PitBull is enabled.
There seems to be no issuses with the older ItemRack 1.991 which is more commonly used (but I haven't tested that version).