I have the same problem that Stephen mentions in his post and have attempted to address it with a variety of methods. I've listed my current setup, a description of my issue, and my attempts to resolve it (without success) below.

My Setup

Umbraco version 7.1.6 assembly: 1.0.5350.25714

Umbraco Contour 3.0.21, 3.0.22WIP, 4.0WIP.

.NET Framework version V4.5

Description of the issue

When creating a form, I am eventually (quickly) unable to save the form after adding/modifying/deleting a field. This affects version 3.0.21 and 3.0.22WIP as of 10/28.

The issue isn't limited to a specific type of form field. Regardless of what form fields I edit (and even if I'm simply saving an existing template), I'm unable to save changes to labels, field removals, etc. Stephen describes the issue well.

Inspecting the header response, I see:

{d: false}

My trials are listed below:

Reinstall 3.0.21

In an attempt to reinstall the plugin, I uninstalled 3.0.21. The uninstall script leaves an umbraco form dll in the bin folder and two references in the web.config file causing the site to break completely with a YSOD on both the frontend and admin panel.

Thankfully, after deleting the dll file and web.config references to umbraco.forms, I was able to access the site again. I reinstalled the current version and the issue persists.

Edit Web Config 

Based on a Google search, I tried adding the following to the web.config file per this article. The modification had no effect on my ability to save the form regardless of which version of contour/forms I had installed.

I also tried the following (which also did not resolve the issue):

Install Build 3.0.22WIP

I have tried reinstalling using the latest build (3.0.22WIP). That specific build does not include the link to Contour form editor in the admin.

I made a guess at the link ({siteurl}/umbraco/contour) and was able to see the form editor. However, the form list was not present in the middle column of the admin panel.

Using that build, I was entirely unable to save any forms at all regardless of whether I used a template or created a form from scratch.

Install Build 4.0WIP.4.0.0-Build.50.zip

I have tried installing Umbraco Forms ver. 4.0WIP using UmbracoForms.package.4.0WIP.4.0.0-Build.50.zip. After installing, the forms link is present in the left navigation of the admin menu. However, there are no options to create a form. What I see, instead, is the following error:

Could not load control: '/umbraco/plugins/umbracocontour/formsdashboard.ascx'. 

Error message: System.Web.HttpException (0x80004005): The file '/umbraco/plugins/umbracocontour/formsdashboard.ascx' does not exist. at System.Web.UI.Util.CheckVirtualFileExists(VirtualPath virtualPath) at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) at System.Web.UI.TemplateControl.LoadControl(VirtualPath virtualPath) at Umbraco.Web.UI.Umbraco.Dashboard.UserControlProxy.OnInit(EventArgs e)

At this point, I can certainly say that I've invested a lot of time encountering errors and trying to troubleshoot a paid plugin. I'm willing to attempt a workaround or a fix, but I'm very frustrated that this is happening.

I understand that the plugin is still in development, but it fails spectacularly at its primary purpose.

What do I have to do to get this plugin to work?