It might be that we need to format the date to remove the time component?
To group by date you'll definitely need to remove the time.
After reading your question properly, I'm not sure the error you're getting is actually because of the grouping. Have you declared any parameters to map to '?'?
I havr this in my code:
labcolltableadapter.Fill(lablogsheet.Labcoll, _fromdtp1.value, _todtp1.value);
reportviewer2.RefreshReport();
I have parameters in dataset Labcoll:
Fill,GetData(labcollect, labcollect1)