DNN 4.9 ... Intermittent error loading PDF files

I am getting intermittent "Object reference not set to object instance" error when uploading PDF files to FCKEditor in DNN 4.9.

Here's the error:

AssemblyVersion: 04.09.00 AbsoluteURL: /.../Providers/HtmlEditorProviders/Fck/fcklinkgallery.aspx
UserAgent: Mozilla / 4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; IEMB3; IEMB3) InnerException: Object reference not set to object instance. FileName:
FileLineNumber: 0 FileColumnNumber: 0 Method: DotNetNuke.UI.UserControls.UrlControl.cmdSave_Click StackTrace: Message: DotNetNuke.Services.Exceptions.PageLoadException: Object reference not set to object instance. ---> System.NullReferenceException: Object reference not set to an object instance. at DotNetNuke.UI.UserControls.UrlControl.cmdSave_Click (object sender, EventArgs e) at System.Web.UI.WebControls.LinkButton.OnClick (EventArgs e) at System.Web.UI.WebControls.LinkButton.RaisePostBackEvent string) System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent (String eventArgument) at System.Web.UI.Page.RaisePostBackEvent (IPostBackEventHandler sourceControl, String eventPostBackEvent) at System.Web.UI.Page.RaisePostBackEvent (IPostBackEventHandler sourceControl, String eventPostBackEvent) at System.Web.UI.Page.RaisePostBackEvent (IPostBackEventHandler sourceControl, String eventPostBackEvent) (NameValueCollection postData) in System.Web.UI.Page.ProcessRequestMain (Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of internal exception stack trace ---

The error comes and goes, the file size doesn't seem to matter, the last time I got it after downloading multiple 1MB files, the third file (600KB) gave me an error.

Please, help!