Unfortunately i found some errors in the calculation:
If you look at the graphik above, all marked rows are being calculated wrong. The second last column (IST kum.) is the calculated cumulative sum (i exported the numbers to excel in order to make it more clear, in the actual report all the cumulative sums are available). The last column is the correct sum calculated in excel.
Has anyone an idea why this formula sometimes works and sometimes not? I couldn't find any patterns...