TFS Server plugin fails after upgrade from TFS2012 to TFS2015 RC - plugins

Question

We have just upgraded one of our servers from TFS2012.2 to TFS2015RC. Everything went "smooth", but we are encountering an issue:
A while ago we wrote a server side plugin for TFS, which listens to the WorkitemChangedEvent. It implements the ISubscriber interface. The following piece of code was working fine before the update:
void ITfsService.UpdateState(int workItemId, string newState)
{
var wi = store.GetWorkItem(workItemId);
wi.State = newState;
wi.Save();
}
After the update, and after recompiling against the TFS2015 dlls, the following error occured:
Failed to process notification: TF237124: Work Item is not ready to save.
Note that none of the workitemtypes has changed, it is the same data.
I tried getting more information out of the error by calling Validate() before saving, this is the output:
Status: InvalidListValue
State: "Resolved, To Be Reviewed"
WIT: Task
Id: 5842
Field: State
However, the state "Resolved, To Be Reviewed" does exists in the list of available states. In the GUI it is perfectly possible to change the state of the item to "Resolved, To Be Reviewed":
What is causing the Save() to fail?

Answer 1

Finally found the cause of this:
Apparently, the Validate() call validates against the whole ProcessConfiguration.xml. Because in TFS2012 there were both AgileProcessConfiguration and CommonProcessConfiguration, there were underlying problems with the workitem states.
After resolving those issues in the correct ProcessConfiguration file, the plugin was working again (and, TFS could also upgrade the Backlog\IterationPlanning features.)