Strange Error - Sales Order Consumption

@operator

Hello,
While using transaction search for downloading salesorders between dates I am getting orders which are existing between only june and july months. For the Sales Order of August and September (as well as pre june06), I am getting the following error
Error code: unexpected_error ,error message:  "An unexpected error occurred."
I am using the following code, I know it is strange as the same code is working for June and July but not for remaining months.

SearchDateField  sdf = new SearchDateField();
sdf.@operator =SearchDateFieldOperator.within;
sdf.operatorSpecified =true;
sdf.searchValue=Convert.ToDateTime(txtDateFrom.Text);  //"9/01/2006"
sdf.searchValueSpecified=true;
sdf.searchValue2=Convert.ToDateTime(txtDateTo.Text); //"9/26/2006"