I have never seen this error before. Is it a new EPiServer 7 site or is it upgraded from previous versions?

it is a new episerver 7 site. is there any error in XFormPageTemplatePage or XformPage.aspx above?

It is a Visual studio Episerver 7 MVC projekt

There error seems to be related to saving the form and that has nothing to do the templates using the form. I suggest that you start a support case to investigate this since I have no clue why you are getting this error.

If i look at the code it is not an MVC project since you are using webforms to create the template for showing the result. Try to connect your db to an empty episerver project based on webforms instead of mvc. 

Thanks for your reply Eric.   It is actually a Mvc project.  as i am new to episerver and Xform, i thought also i am not doing it correctly.

So how should i write above code for mvc project? 

Now i have used following as template

[ContentType(DisplayName = "XFormPageTemplate2", GUID = "4562c1f8-1abd-4f9b-95c7-8b985d31df66", Description = "XFormPageTemplate2")]
public class XFormPageTemplate2 : PageData
{
[Display(
Name = "XFormPageTemplate2",
Description = "XFormPageTemplate2 description",
GroupName = SystemTabNames.Content,
Order = 310)]
[CultureSpecific]
public virtual XForm XFormProperty2 { get; set; }
}

-----------------------------------------------------------

and i am using mvc 4 viewpage with layout (Razor) as follows

@model Totalkredit.EpiServer.Pages.XFormPageTemplate2
@using EPiServer.Core

@{
Layout = "~/Views/Shared/MasterLayout.cshtml";
}

<h1>Xform Template2 page </h1>

@Html.PropertyFor(x => x.XFormProperty2)

----------------------------------------------------------

still getting the same error when i have created a simple form with 2 columns with textbox  and button  

i get the error when i click on save button to save the form. 

Try the below steps:

Page Type:

 [Editable(true)]
        [Display(
            Name = "Content",
            Description = "The Content will be shown in the main content area of the page.",
            GroupName = SystemTabNames.Content,
            Order = 4)]
        public virtual XhtmlString Content { get; set; }


        [Display(GroupName = SystemTabNames.Content, Order = 5)]
        public virtual XForm Form { get; set; }

Model:

  public virtual XForm XformContent { get; set; }

Base Controller class:

  //For XForm Page
        private readonly XFormPageUnknownActionHandler _xformHandler;
        private string _contentId;
        private readonly string _viewDataKeyFormat = "ViewData_{0}";
        private string ViewDataKey
        {
            get
            {
                return string.Format(_viewDataKeyFormat, _contentId);
            }
        }

        public BaseController()
        {
            _xformHandler = new XFormPageUnknownActionHandler();
            _contentId = string.Empty;
        }

  #region"XForm Block"

        protected override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            if (!string.IsNullOrEmpty(_contentId))
            {
                if (TempData[ViewDataKey] != null)
                {
                    ViewData = (ViewDataDictionary)TempData[ViewDataKey];
                }
            }

            base.OnActionExecuting(filterContext);
        }
        [AcceptVerbs(HttpVerbs.Post)]
        public virtual ActionResult Success(XFormPostedData xFormPostedData)
        {
            return RedirectToAction("Index");
        }

        [AcceptVerbs(HttpVerbs.Post)]
        public virtual ActionResult Failed(XFormPostedData xFormPostedData)
        {
            return RedirectToAction("Index");
        }

        [AcceptVerbs(HttpVerbs.Post)]
        public virtual ActionResult XFormPost(XFormPostedData xFormpostedData, string contentId)
        {

            //var formData = new XFormPageHelper().GetXFormData(this, xFormpostedData);
            // var result = XFormActionHelper.DoAction(formData, xFormpostedData, true);
            //return this.RedirectToAction("Index");
            _contentId = contentId;
            return _xformHandler.HandleAction(this);
        }
        #endregion

Controller:

 public ActionResult Index(CustomFormPageType currentPage)
        {

            var currentBlockID = (currentPage as IContent).ContentLink.ID;
            var viewDataKey = string.Format("ViewData_{0}", currentBlockID);

            if (TempData[viewDataKey] != null)
            {
                ViewData = (ViewDataDictionary)TempData[viewDataKey];
            }

            var pageRouteHelper = ServiceLocator.Current.GetInstance<PageRouteHelper>();
           // PageData currentPage1 = pageRouteHelper.Page;
            
            if (currentPage == null)
            {
                var contentLoader = ServiceLocator.Current.GetInstance<IContentLoader>();
                //currentPage = contentLoader.Get<StartPage>(ContentReference.StartPage);
            }
            var actionUri = string.Empty;
            if (currentPage.Form != null && currentPage != null)
            {
                var urlResolver = ServiceLocator.Current.GetInstance<UrlResolver>();
                var pageUrl = urlResolver.GetVirtualPath(currentPage.ContentLink);

                actionUri = string.Format("{0}XFormPost/", pageUrl);
                actionUri = UriSupport.AddQueryString(actionUri, "XFormId", currentPage.Form.Id.ToString());
                actionUri = UriSupport.AddQueryString(actionUri, "failedAction", "Failed");
                actionUri = UriSupport.AddQueryString(actionUri, "successAction", "Success");
                actionUri = UriSupport.AddQueryString(actionUri, "contentId", currentBlockID.ToString());

                
            }

           
            var model = new CustomFormPageViewModel()
            {
               
                XformContent = currentPage.Form
             
            };

           

            return View(model);
        }

View:

   <section id="xform">
                    <div @Html.EditAttributes(m => m.XformContent)>
                @Html.ValidationSummary()
                @using (Html.BeginXForm(Model.XformContent, new { Action = Model.ActionUri }))
                {
                    Html.RenderXForm(Model.XformContent);
                }
            </div>
        </section>

More Information: Refer the URL below

http://blog.nansen.com/2013/03/creating-xform-block-in-episerver-7-mvc.html