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Sorry Stefan, my mistake.
Apparently, I haven't updated the name of the aggregation in all places throughout the presentation. The aggregation is called AggregateCombine, not AggregateUnion.
This query should work:
Select Vertrieb, AggregateCombine(OBJ) From de_verkäufer Group By Vertrieb
A bit weird that this doesn't raise an error but maybe we still have the former name somewhere behind the scenes. I'll check.
Also note, that the Object keyword should only be used if the resulting query has multiple spatial object "columns". In this case, there is only one so you should not use the Object keyword.