Make sure to alias columns with the same name from different tables, so that their names in the resulting query, are unique.
So for your STUKREGsubs2 query, add an alias to one of the STUKLIJSTNUMMER-outputs, like this:
SELECT S.STUKLIJSTNUMMER, SR.STUKLIJSTNUMMER AS STUKLIJSTNUMMER_2, SR.SUB_STUKLIJSTNR
FROM STUKREG AS SR INNER JOIN STUKREGsubs1 AS S ON
SR.STUKLIJSTNUMMER=S.SUB_STUKLIJSTNR
Then, make sure that T.STUKLIJSTNUMMER in your first query, refers to the correct column in STUKREGsubs2 (either STUKLIJSTNUMMER from the STUKREGsubs1-query or STUKLIJSTNUMMER_2 from the STUKREG-table).