TFS 2013 Preview and TF400917: The current configuration is not valid for this feature.

After upgrading to TFS 2013 Preview some of our team projects started to get this error when trying to access backlog:

Some of our projects were successfully upgraded though. So what is to be done in order to fix the error:

1. You need to install Visual Studio 2013. witadmin tool from Visual Studio 2012 can export process config files, but can not import. Make sure you run witadmin from C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE

2. Run
witadmin exportprocessconfig /collection:http://yourTFSserver:8080/tfs/YourCollection /p:YourTeamProject /f:c:\temp\processconfig.xml 

3. Correct processconfig.xml file so for nodes :

• FeedbackRequestWorkItems, 
• FeedbackResponseWorkItems
• RequirementBacklog 
• TaskBacklog 

there exists attributes pluralName and singularName. Like this, for example:
<RequirementBacklog category="Microsoft.RequirementCategory" pluralName="Backlog items" singularName="User Story">
...
</RequirementBacklog>

4. For RequirementBacklog and TaskBacklog there should exist not empty AddPanel sub-node; like this:
<TaskBacklog category="Microsoft.TaskCategory" pluralName="Tasks" singularName="Task">
    <AddPanel>
      <Fields>
        <Field refname="System.Title" />
      </Fields>
    </AddPanel>
...
</TaskBacklog>

5. Run
witadmin importprocessconfig /collection:http://yourTFSserver:8080/tfs/YourCollection /p:YourTeamProject /f:c:\temp\processconfig.xml

6. It might complain about multiple states with the type Complete. Yes, it should be only one. Make sure that this part of processconfig.xml defines only one state as Complete:
<States>
      <State type="Proposed" value="Analysis" />
      <State type="Proposed" value="Proposed" />
      <State type="Proposed" value="Not Started" />
      <State type="InProgress" value="Active" />
      <State type="InProgress" value="Resolved" />
      <State type="Complete" value="Closed" />
</States>

7. Run witadmin importprocessconfig.... again. That should be it.