WebBrowser in thread does not load anything

I have multi-threaded WinForms app, with several threads working with WebBrowser. Here is a part of my code (just a start of the function)

private void createThreads()
    {
     int i = 1;
            foreach (Account acc in Accounts)
            {
                    Thread buyer = new Thread(() => Buy(acc, i)) { ApartmentState = ApartmentState.STA };
                i++;
                buyer.Start();
                Buyers.Add(buyer);
            }
    }
private void Buy(Account ac, int id)
    {
        WebBrowser browser = new WebBrowser();
        browser.ScriptErrorsSuppressed = true;
        Account acc = new Account();
        acc.username = ac.username.ToString();
        acc.password = ac.password.ToString();
        acc.shoes = ac.shoes.ToList();
        while (run)
        {
            if(acc.shoes == null || acc.shoes.Count < 1){return;}
            Product toBuy = acc.shoes[0];
            browser.Navigate(toBuy.url);

            while (browser.Document == null) { Thread.Sleep(25); } //wait for the document to download (at least some data received)
            while (browser.Document.Body == null) { Thread.Sleep(25); } //wait for at least some body to download

Now, the thread never leaves the first while loop, the document is always null. Earlier, I run this in one thread, and instead of Thread.Sleep() I called Application.DoEvents() (as advised in another question here on SO) and it worked fine. However, that seemed to cause AccessViolationException when run on multiple threads, so I changed to Thread.Sleep() here and I call Application.DoEvent() in a Windows.Forms.Timer in the main app thread, called each milliseconds.

Any idea why the browser doesn't load anything and how to fix this?