Basic questions:  Have you run a DB trace?  Can you run the SQL from somewhere else  (outside of PB) against the database?  Have you tried a different driver?

Additional basic questions: When editing this DataWindow in the DataWindow Painter, can you preview (retrieve) successfully? Do other DataWindows retrieve properly, either when running the app or from the DataWindow Painter? Have you tried taking the SQL SELECT statement used as the data source for the failing DataWindow and executing it from the ISQL pane in the PowerBuiulder Database Painter?

Hi John, thanks for your helpfulness.
We have found the problem, I will describe the issue so that it can be useful to others as well.
To answer your questions anyway:
"other DataWindows retrieve properly..." yes, both in the app and in the datawindows painter.
"executing it from the ISQL pane " yes, the select works properly if launched from ISQL pane.

Hi Matt, thanks for your helpfulness.
We have found the problem, I will describe the issue so that it can be useful to others as well.
Tracing the DB from the ODBC driver manager we found: "oobjm 1ae8-7ec EXIT SQLNumResultCols with return code -1 (SQL_ERROR)
HSTMT 0x0096DB60
SWORD * 0x00598EA0
DIAG [S1010] [Microsoft][ODBC Driver Manager] Error in the function sequence. (0)"
We then assumed that the problem was related to some column in the select.
In fact, the problem was related to a particular calculated column.

We found the problem,

a particular column of the select was constructed with a series of concatenations between string fields.
"SELECT ....,
coalesce(column1, ' ') ||'/'|| coalesce(column2,' ')||'/'|| coalesce(column3,' ')||'/'||
coalesce(column4, ' ')||'/'||coalesce(column5, ' ')||'/'|| .... etc AS COMPUTED_STRING"
We used DB2 operator "|| ( double pipe ) " which corresponds to the CONCAT function.
By removing this calculated column everything went well.
We haven't tried replacing the || with the CONCAT function yet, but I think the problem is related to this particular operator. It may not be handled correctly by the ODBC driver.
I suppose we should modify the select with a series of :
SELECT CONCAT(CONCAT('colmun1',' '),'column2') AS COMPUTED_STRING
FROM SYSIBM.SYSDUMMY1;
Alternatively we can concatenate the columns via PB script