I can now open the trade bug save game. This is what I see:
Askia has 4 horses. If I click on his horses (which makes him offer 4 ) and ask what he wants for them, he says no such trade is possible and the number of horses on the table is reset to 0. If I change this to some other number of horses and ask what he wants, the number is reset to 0. OTOH, if I ask for horses and I also place something on the table (such as gold), he again says no such deal is possible, but the number of horses is reset to 4. Washington has 6 horses and essentially the same thing happens, except the number of horses is reset to 5 if I put something on the table when asking for horses.
The title of the thread states that (Strategic) Resource trades are allowed only in multiples of 5 (or less, if the AI has less than 5). However, the save file doesn't demonstrate this because the AI won't accept any trades in this particular situation. It seems that I can propose a trade for any number of horses, but the AI won't accept it. The proposal gets reset to some number other than what I proposed. Is that the bug?
If the bug is that the AI will only accept trades for strategic resources in multiples of 5, then we need a save file where the AI will actually agree to such a trade.