Error in Episerver CMS 5 R2 SP 2 after upgrading to 6 R2

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Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
   EPiServer.Core.PropertyDataCollection.EnsureAllPageTypeProperties() +281
   EPiServer.Core.PropertyDataCollection.InitializeData() +26
   EPiServer.Core.PageData.InitializeData(IList`1 pageLanguages) +20
   EPiServer.DataAccess.PageListDB.ReadPageList(AbstractCommand cmd) +2463
   EPiServer.DataAccess.<>c__DisplayClass11.<LoadSpecificPages>b__10() +428
   EPiServer.DataAccess.<>c__DisplayClass1`1.<Execute>b__0() +58
   EPiServer.DataAccess.DatabaseFactory.Execute(Action`1 method) +55
   EPiServer.DataAccess.DataAccessBase.Execute(Action`1 action) +237
   EPiServer.DataAccess.PageListDB.LoadSpecificPages(Int32[] pageLinks) +78
   EPiServer.LocalPageProvider.GetPages(PageReferenceCollection pageReferences, ILanguageSelector selector) +170
   EPiServer.Core.PageProviderBase.GetScatteredPages(PageReferenceCollection pageRefs, ILanguageSelector selector) +441
   EPiServer.Core.PageProviderBase.GetChildren(PageReference pageLink, ILanguageSelector selector, Int32 startIndex, Int32 maxRows) +92
   EPiServer.DataFactory.GetChildren(PageReference pageLink, ILanguageSelector selector, Int32 startIndex, Int32 maxRows) +261
   EPiServer.Web.UrlSegment.GetPageBySegment(PageReference parent, String urlSegment, String languageBranch, Boolean exactLanguage) +85
   EPiServer.Web.FriendlyUrlRewriteProvider.GetPageBySegments(PageReference rootPageRef, String[] segments, String languageCode) +50
   EPiServer.Web.FriendlyUrlRewriteProvider.GetPageFromStartByPath(String path, LanguageBranch languageBranch) +138
   EPiServer.Web.FriendlyUrlRewriteProvider.ConvertToInternalInternal(UrlBuilder url, Object& internalObject) +413
   EPiServer.Web.FriendlyUrlRewriteProvider.ConvertToInternal(UrlBuilder url, Object& internalObject) +410
   EPiServer.Web.UrlRewriteModule.HttpUrlRewriteToInternal(UrlBuilder url) +313
   EPiServer.Web.UrlRewriteModuleBase.BeginRequestEventHandler(Object sender, EventArgs e) +158
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +68
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

WHAT I DID IS:

I made copy of my local dev website and upgraded it first from 5 R2 SP 2 to 6 then 6 R2 and the upgraded version worked fine.

Then I stopped upgraded website and started to work teh original old version (5 R2 SP 2) by restoring its equivalent DB to have things as they were. But now I suddenly get this strange error and I cant even hit by debug on any entry point in code.

Anyone know how to fix this ?

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Error in Episerver CMS 5 R2 SP 2 after upgrading to 6 R2

Server Error in '/' Application.

Object reference not set to an instance of an object.