Cannot buy stockpile items anymore – even if they are available!

It really pisses me off!
I was just playing “Assassin’s Creed 3” up to sequence 9. Right now, I am exploring the map and making money by buying pelts from Myriam and selling them via a land or naval convoy. In the meantime, I am trying to produce as many goods as possible. But there seems to be a certain point, where there is some kind of horrible bug in the game.
So, I was trying to buy some new pelts from Myriam or sand from Norris, but I just couldn’t buy it. For example, there were 7 sand available, that is it said 0/7. I filled it up to 7/7 and pressed enter, but no sand was added to my inventory and it still said 0/7! However, the amount of money was subtracted! Same for bear pelts and some other stuff!
What is that?! That problem now occurs for the seconed time, after I have reinstalled “Assassin’s Creed 3” and played the game again – right from the beginning!
I don’t know how to deal with that problem or if there is any solution to it. I just found some sites, where the same problem is described, but there is no solution to it. Here is an example:
So, maybe I can find a solution here.
I know that there are some suggestions on how to solve that problem, but nothing works for me. Reducing my items below the amount of 1500 does not work, as well as accomplishing some missions. Also, many people do not seem to understand the question properly. That’s why I am posting this here, hoping to finally get a working solution.
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