Almost an year ago I wrote here about two problems: the first was that merchant often served twice or more one request, decrementing (or incrementing if sell) proportionally my credit; the second was that system logged me out without cause, saying "already logged".
Both problems were random.
Now I can say that both evanished when I reinstalled Windows, that the second reappeared after the reinstallation of "Trend Micro Titanium" antivirus and evanished again after reinstallation of Windows without installation of that antivirus.
This to help anyone had this kind of problem.