Web forms for marketers redirect assumes the success page is relative to the form item in the tree (template: /sitecore/templates/Web Forms for Marketers/Form). When this is not the case, wffm builds a redirect URL which is not valid. Since the success page is defined using a general link field, we don't have a Guid to lookup the actual success …

gistfile1.cs

  /*
   Extends: Sitecore.Form.Core.Pipelines.FormSubmit
   
   forms.config needs to be updated to:
  
      <successAction>
	<processor type="MyCustom.Pipelines.SuccessRedirect, MyLibrary"/>
        <processor type="Sitecore.Form.Core.Pipelines.FormSubmit.FormatSuccessMessage, Sitecore.Forms.Core"/>
      </successAction>
  */


  public class SuccessRedirect : ClientPipelineArgs
	{
		public void Process(SubmitSuccessArgs args)
		{
			Assert.IsNotNull(args, "args");
			if (args.Form == null)
			{
				Uri uri = null;
				if (Uri.TryCreate(args.Result, UriKind.RelativeOrAbsolute, out uri))
				{
					bool isUriValid = uri != null && !string.IsNullOrEmpty(uri.PathAndQuery);
					if (isUriValid)
					{
						args.AbortPipeline();
						WebUtil.Redirect(args.Result, false);
					}
				}
			}
			else
			{
				if (args.Form.SuccessRedirect)
				{
					var url = GetRedirectUrl(args);
					if (!string.IsNullOrEmpty(url))
					{
						args.AbortPipeline();
						WebUtil.Redirect(url, false);	
					}
				}
			}
		}

		private string GetRedirectUrl(SubmitSuccessArgs args)
		{
			string url = string.Empty;
			var path = "/sitecore/content" + args.Form.SuccessPage.Url;	// don't love the hard-coded path here. Other ideas?

			if (!string.IsNullOrEmpty(path))
			{
				var item = Sitecore.Context.Database.Items[path];
				if (item != null)
				{
					url = LinkManager.GetItemUrl(item);
				}
			}

			return url;
		}
	}

Hi @PetersonDave, Does this mean this is the reason for the reload of the form page that occurs on submit before actually forwarding the request to the success page?