Ошибки компилятора VC2008, открывающиеся sbr файлы (C2418 C1903 C2471)

Используйте GROUP BY и SUM():

SELECT
  q.Monat
, q.Jahr
, q.Project
, SUM(q.cum_cnt_of_tasks) SumCnt_of_tasks
FROM
(
SELECT 
    *, 
    SUM(a.cnt)  OVER (PARTITION BY Project ORDER BY Jahr,Monat,Project RANGE 
        UNBOUNDED PRECEDING) as cum_cnt_of_tasks
FROM 
    (
    SELECT 
    count(distinct [StoryID])as cnt
    , datepart(month,[Story_Resolutiondate]) as Monat
    , datepart(year,[Story_Resolutiondate]) as Jahr
    ,v.ProjectID as  Project
    FROM [Story]s 
    join [Epic]e on s.EpicID=e.EpicID
    join [Project]v on e.ProjectID=v.ProjectID
    group by  datepart(month,[Story_Resolutiondate]) 
             ,datepart(year,[Story_Resolutiondate])
             ,v.ProjectID    
  ) AS a 
)q
GROUP BY   q.Monat
         , q.Jahr
         , q.Project
         , q.cum_cnt_of_tasks

ОБНОВЛЕНИЕ:

Попробуйте использовать следующий запрос:

SELECT 
  CASE 
    WHEN t.Jahr = 2017 THEN 1
    ELSE MAX(t.Monat)
  END AS Monat     
, CASE 
    WHEN t.Jahr = 2017 THEN 2019
    ELSE t.Jahr
  END AS Jahr
, t.Project
, COUNT(t.cum_cnt_of_tasks) Cnt
FROM @Table t
GROUP BY 
  t.Jahr
, t.Project

DECLARE @Table TABLE
(
    Monat  INT,
    Jahr INT,
    Project INT,
    cum_cnt_of_tasks INT
)

INSERT INTO @Table
(
    Monat,
    Jahr,
    Project,
    cum_cnt_of_tasks
)
VALUES
  (   6,     2017,        259052,               1)
, (   7,     2017,        259052,               2)
, (   8,     2017,        259052,               3)
, (   9,     2017,        259052,               4)
, (   1,     2019,        255403,               4) 
, (   2,     2019,        255405,               2)

ВЫХОД:

Monat   Jahr    Project   Cnt
1       2019    255403     1
2       2019    255405     1
1       2019    259052     4