Hi oksummer,
Got this error because your virtual directory create not correct. So editor can not find the folder under your site.
You can test it with the simple code below. If the virtual directory is under your site, then when page load, it will create the 'test.txt' under the virtual directory. If you get error with it, that means under your site has not the "WebsitesNew" virtual directory.
protected override void OnLoad(EventArgs e)
{
System.IO.File.Create(Server.MapPath("~/WebsitesNew/test.txt"));
}
Regards,
ken