Please post a new question, with code that actually demonstrates the problem described, and we will be happy to look at it.

The code you posted here works on every version of SQL Anywhere from 9 to 16...

@@VERSION,artid_head,artnr_head,cc_plan_customer
'9.0.2.3951',2,5,'333 / X'

@@VERSION,artid_head,artnr_head,cc_plan_customer
'10.0.1.4310',2,5,'333 / X'

@@VERSION,artid_head,artnr_head,cc_plan_customer
'11.0.1.2960',2,5,'333 / X'

@@VERSION,artid_head,artnr_head,cc_plan_customer
'12.0.1.3994',2,5,'333 / X'

@@VERSION,artid_head,artnr_head,cc_plan_customer
'16.0.0.1691',2,5,'333 / X'

Here is the "reproducible"...

CREATE TABLE tblArt (
   ID         INTEGER,
   artid_head INTEGER );
INSERT tblArt VALUES ( 1, 2 );
CREATE TABLE tblBen (
   ID         INTEGER,
   artnr_head INTEGER );
INSERT tblBen VALUES ( 1, 5 );
CREATE TABLE customer (
   k_namn VARCHAR ( 10 ),
   k_kod  INTEGER );
INSERT customer VALUES ( 'X', 333 );
CREATE PROCEDURE sp_get_info ( p1 INTEGER, p2 INTEGER, p3 INTEGER )
BEGIN
   SELECT 111 AS customer,
          222 AS period,
          333 AS lev_customer,
          -444 AS qty,
          555 AS best,
          666 AS reserv,
          777 AS artnr,
          888 AS datum;
END;
Select    @@VERSION, 
          tblArt.artid_head,
          tblBen.artnr_head,

(SELECT FIRST 
                STRING(ct_disp.customer)  + ' / ' + (select customer.k_namn from customer where customer.k_kod =ct_disp.customer) 
            FROM
                (SELECT 
                    period as period,
                    lev_customer as customer,
                    qty + SUM(best - reserv) OVER (PARTITION BY artnr ORDER BY datum ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) as qty
                FROM 
                            sp_get_info(artnr_head, artid_head, 1)) AS ct_disp
            WHERE
                ct_disp.qty < 0
            ORDER BY
                ct_disp.period) as cc_plan_customer

FROM 
        tblArt,
        tblBen
    WHERE
        tblArt.ID = tblBen.ID;