Also, be aware that "Index" and "Month" are reserved words and shouldn't be used for object names.
Plus, "Index" and "Month" aren't very descriptive.
Basic troubleshooting (no pun intended):
Start with one field for the select query.
Code:
SELECT DISTINCT CLng([DatumAkt])&"_"&Trim([collectorid]) AS [Index] FROM q_kontakt_sum;
If that executes properly (no error), then add 1 field:
Code:
SELECT DISTINCT
CLng([DatumAkt])&"_"&Trim([collectorid]) AS [Index],
CLng([Month])&"_"&Trim([collectorid]) AS Index2
FROM q_kontakt_sum;
Keep adding 1 field until you determine which field causes the error.
Once you determine which field is causing the error, you can fix it and move on to the next error (if any).