Exception Message: The given key was not present in the dictionary. (type KeyNotFoundException)
Exception Stack Trace: at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
at Microsoft.TeamFoundation.Server.WebAccess.Agile.Models.WorkItemSource.<>c__DisplayClassa.<GetCompleteWorkItemData>b__6(IDataRecord dataRecord)
As this is a known issue, solution is described here:
But... it didn't help. Yes, there was inconsistency and 2 possible work items states mapped to Complete board state, but after fixing CommonProcessConfig.xml and leaving only User Story work item in Requirements categories - error still kept coming.
It looks like if there are work items which have the "invalid" state, the board is not getting rebuilt. So, in this unlucky case the fastest fix is to create a new area, and make it the only and default area for backlog.Which practically makes the backlog (and the board) empty.
Correct this now, and assign relevant states for the board columns.