SET @typ = 30;
SET @ono = 28; -- test with 29, 38, 71, 82 and 462 where the error occurs as well

SELECT sorder.*,
      cust.name,
      cust.curr_code,
      cust.address,
      loc.location_name,
      cust.discount,
      stype.sales_type,
      stype.id AS sales_type_id,
      stype.tax_included,
      stype.factor,
       ship.shipper_name,
      tax_group.name AS tax_group_name,
      tax_group.id AS tax_group_id,
      cust.tax_id,
      sorder.alloc,
      IFNULL(allocs.ord_allocs, 0)+IFNULL(inv.inv_allocs ,0) AS sum_paid,
      sorder.prep_amount>0 AS prepaid
    FROM 1_sales_orders sorder
            LEFT JOIN (SELECT trans_no_to, SUM(amt) ord_allocs FROM 1_cust_allocations
                WHERE trans_type_to=30 AND trans_no_to=@ono GROUP BY trans_no_to)
                 allocs ON sorder.trans_type=30 AND allocs.trans_no_to=sorder.order_no
            LEFT JOIN (SELECT order_, SUM(alloc) inv_allocs FROM 1_debtor_trans 
                WHERE TYPE=10 AND order_=@ono  GROUP BY order_)
                 inv ON sorder.trans_type=30 AND inv.order_=sorder.order_no
            LEFT JOIN 1_shippers ship ON  ship.shipper_id = sorder.ship_via,
      1_debtors_master cust,
      1_sales_types stype, 
      1_tax_groups tax_group, 
      1_cust_branch branch,
      1_locations loc
    WHERE sorder.order_type=stype.id
        AND branch.branch_code = sorder.branch_code
        AND branch.tax_group_id = tax_group.id
        AND sorder.debtor_no = cust.debtor_no
        AND loc.loc_code = sorder.from_stk_loc
        AND sorder.trans_type = @typ
        AND sorder.order_no = @ono;