C# Webbrowser Control Access Denied after back to previous URL

I have a problem with simple code as below. I have a loop through span tag on page xxx.pl. During search through second loop i have to navigate to another url in third loop (on same server and same catalog) xxx.pl/yyy and back to previous url xxx.pl. After that i got error massage at

if (el.GetAttribute("className") == "account-block-subtext") line.

Error massage is:

  System.UnauthorizedAccessException
  HResult=0x80070005
  Message=Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED))
  Source=System.Windows.Forms

Any suggestions? In my opinion some operation change el object so i can`t access to it after exiting third loop. Maby session data are lost? Code is below:

foreach (HtmlElement el in doc.GetElementsByTagName("span"))
   {
                          
       if (el.GetAttribute("className") == "account-block-subtext")
       {

       foreach (HtmlElement elchild in el.Children)
       {

       if (elchild.TagName == "A")
        {
       string alink = "";
       try { alink = elchild.GetAttribute("href"); } catch { alink = ""; }
       if (alink.Contains("/user/bbb.php"))
       {
       // klik alink
       elchild.InvokeMember("Click");

       webBrowser1.Navigate("xxx.pl/yyy");
          
      while (webBrowser1.ReadyState != WebBrowserReadyState.Complete)
      {
        System.Windows.Forms.Application.DoEvents();
        System.Threading.Thread.Sleep(500);
        }
        //Debug.WriteLine(webBrowser1.Url.AbsoluteUri);

        foreach (HtmlElement cashmail in doc.GetElementsByTagName("span"))
        {
         //  Debug.WriteLine(">" + cashmail.GetAttribute("className"));

      if (cashmail.GetAttribute("className") == "to_read")
      {
       Debug.WriteLine("cashmail");
       }
      }
                                //doc = doc_temp;
        }
                            
        if (alink.Contains("/shops/pppp.php"))
        {
          // klik alink
         Debug.WriteLine(alink);
         }
         alink = "";

         }
         }
         }
         }