Hi Galina,
a LEFT OUTER JOIN returns all the values belonging to the "left" table even if the join condition doesn't match on the "right" table - in this case a NULL value will be present in any column coming from the "right" table.
HANA's Calculation engine considers NULL value as a particular case (and honestly there is a difference between a VARCHAR with length 0 and a NULL)...you should correct your filter in something similar to
WHERE (UEPOS_KONDM !=''15" OR UEPOS_KONDM IS NULL)
Cheers,
Riccardo