Again, this is just a shot in the dark. I don't know if this will fix your issue.
You can wrap your first query in parentheses making it a subquery and 'select' it again like this:
Code:
SELECT *
FROM (SELECT ( Format([KwartTijd], 'ddddd hh') ) AS DATUM,
Sum(PS_Hautrage.Vermogen) AS SomHautrage,
Sum(PS_Buggenhout.Vermogen) AS SomBuggenhout,
Sum(PS_Mainvault.Vermogen) AS SomMainvault,
Sum(PS_AssePastinaken.Vermogen) AS SomAssePastinaken,
Sum(PS_BrakelKerkhofStrt.Vermogen) AS SomBrakelKerkhofstraat,
Sum(PS_AthChaussee.Vermogen) AS SomAthChaussee
FROM ((PS_AthChaussee
INNER JOIN PS_BrakelKerkhofStrt
ON PS_AthChaussee.Id = PS_BrakelKerkhofStrt.Id)
INNER JOIN PS_AssePastinaken
ON ( PS_AthChaussee.Id = PS_AssePastinaken.Id )
AND ( PS_AthChaussee.Id = PS_AssePastinaken.Id ))
INNER JOIN (((DatumTijd
INNER JOIN PS_Hautrage
ON DatumTijd.Id = PS_Hautrage.Id)
INNER JOIN PS_Buggenhout
ON ( PS_Hautrage.Id = PS_Buggenhout.Id )
AND ( PS_Hautrage.Id = PS_Buggenhout.Id )
AND ( PS_Hautrage.Id = PS_Buggenhout.Id )
AND ( DatumTijd.Id = PS_Buggenhout.Id ))
INNER JOIN PS_Mainvault
ON ( PS_Hautrage.Id = PS_Mainvault.Id )
AND ( PS_Buggenhout.Id = PS_Mainvault.Id )
AND ( DatumTijd.Id = PS_Mainvault.Id ))
ON ( PS_BrakelKerkhofStrt.Id = DatumTijd.Id )
AND ( PS_AthChaussee.Id = DatumTijd.Id )
AND ( PS_AssePastinaken.Id = DatumTijd.Id )
AND ( PS_AthChaussee.Id = PS_Buggenhout.Id )
AND ( PS_BrakelKerkhofStrt.Id = PS_Buggenhout.Id )
AND ( PS_AssePastinaken.Id = PS_Buggenhout.Id )
AND ( PS_AssePastinaken.Id = PS_Hautrage.Id )
AND ( PS_AthChaussee.Id = PS_Hautrage.Id )
AND ( PS_BrakelKerkhofStrt.Id = PS_Hautrage.Id )
AND ( PS_AssePastinaken.Id = PS_Hautrage.Id )
AND ( PS_BrakelKerkhofStrt.Id = PS_Mainvault.Id )
AND ( PS_AssePastinaken.Id = PS_Mainvault.Id )
AND ( PS_AthChaussee.Id = PS_Mainvault.Id )
GROUP BY ( Format([KwartTijd], 'ddddd hh') )) AS sbqry
ORDER BY DATUM;
or you could save your first query and call it from another query like this:
Code:
SELECT *
FROM saved_subquery_name
ORDER BY DATUM;
My reasoning for this is you have a crazy FROM clause there with all those JOINs/ANDs. I wonder if access is just having a hard time processing and sorting. This way you force access to collect the data FIRST in a subquery, and then sorting them second after all the hard work had been done. I've found sometimes the access sql engine needs some hand holding when the sql start to get complex.
I could be wrong but I believe that if you save your subqueries separately (like example 2) it's supposed to be better for performance.
(edit)
fyi, it depends on the circumstances but if you can get away with it I think it's generally recommended not to do the sorting in the query itself and just sort it in the form or report instead.