Very tricky: Can't update a sequential list SQL-92 ( Filemaker)
Here is another approach, which, however, is likely to perform worse than the other suggestion:

UPDATE
  Beleg
SET
  SortID = (
    SELECT
      COUNT(*)
    FROM
      Beleg AS Beleg2
    WHERE
      Year ( Beleg2.Valuta ) = Year ( Beleg.Valuta )
      AND Beleg2.Ursprungskonto = Beleg.Ursprungskonto
      AND Beleg2.SortID &lt;= Beleg.SortID
  )
WHERE
  Year ( Valuta ) = 2016
  AND Ursprungskonto = 1210
;


On the plus side, not only does this method reset SortID values in a single step, it can also do that for multiple (Year(Valuta), Ursprungskonto) subsets at once, if need be. That is, you can specify multiple values for either attribute in the main WHERE clause, e.g. like this:

WHERE
  Year ( Valuta ) BETWEEN 2011 AND 2016
  AND Ursprungskonto IN (1210, 2101, 1012)


or even just remove the WHERE clause altogether – and each subset will be re-numerated separately from the others, all with the single query.

Important note: this method assumes that SortID values are unique across any single (Year(Valuta), Ursprungskonto) subset.