Open Draft in OWA with EWS (WebClientEditFormQueryString)

Hello,

I'm trying to open a draft in OWA through C# WebBrowser. Everything works fine with Exchange 2010, but I've a problem with Office 365. The draft is opened correctly, but in read-only mode. Then I've to click "Bearbeitung fortsetzen" (maybe "continue edit" in english?).

I'm using the following code to determine the URL:

      PropertySet propSet = new PropertySet();
      propSet.Add(ItemSchema.Id);
      propSet.Add(ItemSchema.WebClientReadFormQueryString);
      propSet.Add(ItemSchema.WebClientEditFormQueryString);
      Item item = Item.Bind(_service, e.Id, propSet);

      if (item.WebClientEditFormQueryString.StartsWith("http"))
        _strPreparedURL = item.WebClientEditFormQueryString;
      else
        _strPreparedURL = _strOWAUrl + "/" + item.WebClientEditFormQueryString;
...

Is this a bug, or does anyone have a working solution for my problem?

Note: I've already asked this question in the Office365 forum, but they've redirected my to this forum. (https://community.office365.com/en-us/f/158/t/401603)