YEEHA, think I've got it!!!
Thanks to ScaryGreenGhosts's post I've tried to BARTER instead of TRADE with the vendor and voila - after this the game saves like normal! YOOOHOO!!
Oh, and btw, I think it doesn't matter which trader in which act you do it with. I for example have just traded with Volo in my camp in act 1 and it worked!
Well, seems like the trade is broken for now, so I'll just make sure that I barter...
UPD: And also seems like you have to initiate the trade not trough a dialog's phrase but with the "Y" button (playing on a controller, don't know how it is on a K+M), otherwise it's 516 after the barter again
